Prevent undefined behaviour during process exit

[swebb2066]

When some background threads call LogLog::debug on exit which can be after the static LogLogPrivate pointer is deleted by the onexit processing chain.

PR #219 was not sufficient to fix this because the raw pointer held by std::unique_pointer is not zeroed by the std::unique_pointer destructor.

[swebb2066]

For the record, this PR fixes the following deadlock in multithreadtest, when using XMLSocketAppender to connect to a TCP port that does not have a server process listening.
`(gdb) thread apply all bt

Thread 5 (Thread 0x7f4287fff640 (LWP 7247) "multithreadtest"):
#0 futex_wait (private=0, expected=2, futex_word=0x561a8dbb2c08) at ../sysdeps/nptl/futex-internal.h:146
#1 __GI___lll_lock_wait (futex=futex@entry=0x561a8dbb2c08, private=0) at ./nptl/lowlevellock.c:49
#2 0x00007f428e14805d in lll_mutex_lock_optimized (mutex=0x561a8dbb2c08) at ./nptl/pthread_mutex_lock.c:48
#3 ___pthread_mutex_lock (mutex=0x561a8dbb2c08) at ./nptl/pthread_mutex_lock.c:128
#4 0x00007f428e67e68e in log4cxx::helpers::AppenderAttachableImpl::appendLoopOnAppenders(std::shared_ptrlog4cxx::spi::LoggingEvent const&, log4cxx::helpers::Pool&) () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#5 0x00007f428e6e7403 in log4cxx::Logger::callAppenders(std::shared_ptrlog4cxx::spi::LoggingEvent const&, log4cxx::helpers::Pool&) const () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#6 0x00007f428e6e8170 in log4cxx::Logger::addEvent(std::shared_ptrlog4cxx::Level const&, std::__cxx11::basic_string<char, std::char_traits, std::allocator >&&, log4cxx::spi::LocationInfo const&) const () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#7 0x0000561a8c6c2944 in multithread_logger(int) ()
#8 0x00007f428e3d4253 in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#9 0x00007f428e144ac3 in start_thread (arg=) at ./nptl/pthread_create.c:442
#10 0x00007f428e1d6a40 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

Thread 4 (Thread 0x7f428c9a8640 (LWP 7246) "multithreadtest"):
#0 __futex_abstimed_wait_common64 (private=128, cancel=true, abstime=0x0, op=265, expected=7244, futex_word=0x7f428d9aa910) at ./nptl/futex-internal.c:57
#1 __futex_abstimed_wait_common (cancel=true, private=128, abstime=0x0, clockid=0, expected=7244, futex_word=0x7f428d9aa910) at ./nptl/futex-internal.c:87
#2 __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x7f428d9aa910, expected=7244, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=128) at ./nptl/futex-internal.c:139
#3 0x00007f428e146624 in __pthread_clockjoin_ex (threadid=139923820291648, thread_return=0x0, clockid=0, abstime=0x0, block=) at ./nptl/pthread_join_common.c:105
#4 0x00007f428e3d42c7 in std::thread::join() () from /lib/x86_64-linux-gnu/libstdc++.so.6
#5 0x00007f428e7572d9 in log4cxx::net::SocketAppenderSkeleton::close() () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#6 0x00007f428e67ea71 in log4cxx::helpers::AppenderAttachableImpl::removeAllAppenders() () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#7 0x00007f428e6c8c84 in log4cxx::Hierarchy::~Hierarchy() () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#8 0x00007f428e6cdaba in std::_Sp_counted_ptr<log4cxx::Hierarchy*, (__gnu_cxx::_Lock_policy)2>::_M_dispose() () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#9 0x00007f428e6a3745 in log4cxx::spi::DefaultRepositorySelector::~DefaultRepositorySelector() () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#10 0x00007f428e68386a in log4cxx::helpers::APRInitializer::~APRInitializer() () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#11 0x00007f428e0f5495 in __run_exit_handlers (status=0, listp=0x7f428e2c9838 <__exit_funcs>, run_list_atexit=run_list_atexit@entry=true, run_dtors=run_dtors@entry=true) at ./stdlib/exit.c:113
#12 0x00007f428e0f5610 in __GI_exit (status=) at ./stdlib/exit.c:143
#13 0x00007f428e149ee8 in __pthread_once_slow (once_control=0x561a8c6ce070 <multithread_logger(int)::exiting>, init_routine=0x7f428e3d2d50 <__once_proxy>) at ./nptl/pthread_once.c:116
#14 0x0000561a8c6c2a0b in multithread_logger(int) ()
#15 0x00007f428e3d4253 in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#16 0x00007f428e144ac3 in start_thread (arg=) at ./nptl/pthread_create.c:442
#17 0x00007f428e1d6a40 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

Thread 3 (Thread 0x7f428d1a9640 (LWP 7245) "multithreadtest"):
#0 futex_wait (private=0, expected=2, futex_word=0x561a8dbb2c08) at ../sysdeps/nptl/futex-internal.h:146
#1 __GI___lll_lock_wait (futex=futex@entry=0x561a8dbb2c08, private=0) at ./nptl/lowlevellock.c:49
#2 0x00007f428e14805d in lll_mutex_lock_optimized (mutex=0x561a8dbb2c08) at ./nptl/pthread_mutex_lock.c:48
#3 ___pthread_mutex_lock (mutex=0x561a8dbb2c08) at ./nptl/pthread_mutex_lock.c:128
#4 0x00007f428e67e68e in log4cxx::helpers::AppenderAttachableImpl::appendLoopOnAppenders(std::shared_ptrlog4cxx::spi::LoggingEvent const&, log4cxx::helpers::Pool&) () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#5 0x00007f428e6e7403 in log4cxx::Logger::callAppenders(std::shared_ptrlog4cxx::spi::LoggingEvent const&, log4cxx::helpers::Pool&) const () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#6 0x00007f428e6e8170 in log4cxx::Logger::addEvent(std::shared_ptrlog4cxx::Level const&, std::__cxx11::basic_string<char, std::char_traits, std::allocator >&&, log4cxx::spi::LocationInfo const&) const () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#7 0x0000561a8c6c2944 in multithread_logger(int) ()
#8 0x00007f428e3d4253 in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#9 0x00007f428e144ac3 in start_thread (arg=) at ./nptl/pthread_create.c:442
#10 0x00007f428e1d6a40 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

Thread 2 (Thread 0x7f428d9aa640 (LWP 7244) "multithreadtest"):
#0 futex_wait (private=0, expected=2, futex_word=0x561a8dbb2948) at ../sysdeps/nptl/futex-internal.h:146
#1 __GI___lll_lock_wait (futex=futex@entry=0x561a8dbb2948, private=0) at ./nptl/lowlevellock.c:49
#2 0x00007f428e148002 in lll_mutex_lock_optimized (mutex=0x561a8dbb2948) at ./nptl/pthread_mutex_lock.c:48
#3 ___pthread_mutex_lock (mutex=0x561a8dbb2948) at ./nptl/pthread_mutex_lock.c:93
#4 0x00007f428e6f1c10 in log4cxx::helpers::LogLog::debug(std::__cxx11::basic_string<char, std::char_traits, std::allocator > const&) () from /home/stephen/build/logging-log4cxx/src/main/cpp/libl--Type for more, q to quit, c to continue without paging--c
og4cxx.so.15
#5 0x00007f428e759745 in log4cxx::net::SocketAppenderSkeleton::monitor() () from /home/stephen/build/logging-log4cxx/src/main/cpp/liblog4cxx.so.15
#6 0x00007f428e3d4253 in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#7 0x00007f428e144ac3 in start_thread (arg=) at ./nptl/pthread_create.c:442
#8 0x00007f428e1d6a40 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

Thread 1 (Thread 0x7f428df20b80 (LWP 7243) "multithreadtest"):
#0 __futex_abstimed_wait_common64 (private=128, cancel=true, abstime=0x0, op=265, expected=7245, futex_word=0x7f428d1a9910) at ./nptl/futex-internal.c:57
#1 __futex_abstimed_wait_common (cancel=true, private=128, abstime=0x0, clockid=0, expected=7245, futex_word=0x7f428d1a9910) at ./nptl/futex-internal.c:87
#2 __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x7f428d1a9910, expected=7245, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=128) at ./nptl/futex-internal.c:139
#3 0x00007f428e146624 in __pthread_clockjoin_ex (threadid=139923811898944, thread_return=0x0, clockid=0, abstime=0x0, block=) at ./nptl/pthread_join_common.c:105
#4 0x00007f428e3d42c7 in std::thread::join() () from /lib/x86_64-linux-gnu/libstdc++.so.6
#5 0x0000561a8c6c2237 in MultithreadTest::testMultithreadedLoggers() ()
#6 0x0000561a8c6c1d42 in void LogUnit::runTest(abts_case*, void (MultithreadTest::)()) ()
#7 0x0000561a8c6c4630 in abts_run_test(abts_suite, char const*, void ()(abts_case, void*), void*) ()
#8 0x0000561a8c6c5c31 in abts_run_suites(abts_suite*) ()
#9 0x0000561a8c6c0bd0 in main ()
(gdb)
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