tevent: fix access after free in tevent_common_check_signal()

This was reported by Pavel Březina <pbrezina@redhat.com>:

    We found a crash in SSSD when a tevent signal is freed in its handler, tevent
    than crashes when it access siginfo.

    sig_info is freed in signal destructor:

    > #ifdef SA_SIGINFO
    >         if (se->sa_flags & SA_SIGINFO) {
    >             if (sig_state->sig_info[se->signum]) {
    >                 talloc_free(sig_state->sig_info[se->signum]);
    >                 sig_state->sig_info[se->signum] = NULL;
    >             }
    >         }
    > #endif

    (gdb) bt
    #0  0x00007f5d4d86cc74 in tevent_signal_destructor (se=0x7f5d5370f920) at
    ../tevent_signal.c:213
    samba-team#1  0x00007f5d4d65f233 in _talloc_free_internal () from /lib64/libtalloc.so.2
    samba-team#2  0x00007f5d4d6593a3 in _talloc_free () from /lib64/libtalloc.so.2
    samba-team#3  0x00007f5d4342f3d4 in proxy_child_init_done (subreq=0x7f5d5370f600) at
    src/providers/proxy/proxy_auth.c:436
    samba-team#4  0x00007f5d4d86b0c2 in _tevent_req_error (req=req@entry=0x7f5d5370f600,
    error=error@entry=5, location=location@entry=0x7f5d43433010
    "src/providers/proxy/proxy_auth.c:356")
        at ../tevent_req.c:167
    samba-team#5  0x00007f5d4342ef5e in pc_init_sig_handler (ev=<optimized out>,
    sige=<optimized out>, signum=<optimized out>, count=<optimized out>,
    __siginfo=<optimized out>, pvt=<optimized out>)
        at src/providers/proxy/proxy_auth.c:356
    samba-team#6  0x00007f5d4d86d48c in tevent_common_check_signal (ev=0x7f5d536de670) at
    ../tevent_signal.c:428
    samba-team#7  0x00007f5d4d86f28c in epoll_event_loop (tvalp=0x7fff7b568490,
    epoll_ev=0x7f5d536de8b0) at ../tevent_epoll.c:647
    samba-team#8  epoll_event_loop_once (ev=<optimized out>, location=<optimized out>) at
    ../tevent_epoll.c:926
    samba-team#9  0x00007f5d4d86d7d7 in std_event_loop_once (ev=0x7f5d536de670,
    location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent_standard.c:114
    samba-team#10 0x00007f5d4d869fbd in _tevent_loop_once (ev=ev@entry=0x7f5d536de670,
    location=location@entry=0x7f5d50faedc3 "src/util/server.c:668") at
    ../tevent.c:530
    samba-team#11 0x00007f5d4d86a15b in tevent_common_loop_wait (ev=0x7f5d536de670,
    location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent.c:634
    samba-team#12 0x00007f5d4d86d777 in std_event_loop_wait (ev=0x7f5d536de670,
    location=0x7f5d50faedc3 "src/util/server.c:668") at ../tevent_standard.c:140
    samba-team#13 0x00007f5d50f96863 in server_loop (main_ctx=0x7f5d536dfac0) at
    src/util/server.c:668
    samba-team#14 0x00007f5d5180aa42 in main (argc=8, argv=<optimized out>) at
    src/providers/data_provider_be.c:2909

    But then it is accessed again in tevent_common_check_signal:

    > #ifdef SA_SIGINFO
    >         if (clear_processed_siginfo) {
    >             uint32_t j;
    >             for (j=0;j<count;j++) {
    >                 uint32_t ofs = (counter.seen + j)
    >                     % TEVENT_SA_INFO_QUEUE_COUNT;
    >                 memset((void*)&sig_state->sig_info[i][ofs],
    >                     '\0',
    >                     sizeof(siginfo_t));
    >             }
    >         }
    > #endif

    (gdb) bt
    #0  0x00007fd7ba400505 in memset (__len=<optimized out>, __ch=<optimized out>,
    __dest=<optimized out>) at /usr/include/bits/string3.h:84
    samba-team#1  tevent_common_check_signal (ev=0x7fd7bfddf670) at ../tevent_signal.c:459
    samba-team#2  0x00007fd7ba40228c in epoll_event_loop (tvalp=0x7fff85536430,
    epoll_ev=0x7fd7bfddf8b0) at ../tevent_epoll.c:647
    samba-team#3  epoll_event_loop_once (ev=<optimized out>, location=<optimized out>) at
    ../tevent_epoll.c:926
    samba-team#4  0x00007fd7ba4007d7 in std_event_loop_once (ev=0x7fd7bfddf670,
    location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent_standard.c:114
    samba-team#5  0x00007fd7ba3fcfbd in _tevent_loop_once (ev=ev@entry=0x7fd7bfddf670,
    location=location@entry=0x7fd7bdb417c3 "src/util/server.c:668") at
    ../tevent.c:530
    samba-team#6  0x00007fd7ba3fd15b in tevent_common_loop_wait (ev=0x7fd7bfddf670,
    location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent.c:634
    samba-team#7  0x00007fd7ba400777 in std_event_loop_wait (ev=0x7fd7bfddf670,
    location=0x7fd7bdb417c3 "src/util/server.c:668") at ../tevent_standard.c:140
    samba-team#8  0x00007fd7bdb29343 in server_loop (main_ctx=0x7fd7bfde0ac0) at
    src/util/server.c:668
    samba-team#9  0x00007fd7be39ca42 in main (argc=8, argv=<optimized out>) at
    src/providers/data_provider_be.c:2909

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11308

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Jun  2 21:02:11 CEST 2015 on sn-devel-104

lib/tevent/tevent_signal.c

@@ -457,7 +457,7 @@ int tevent_common_check_signal(struct tevent_context *ev)
 		}
 
 #ifdef SA_SIGINFO
-		if (clear_processed_siginfo) {
+		if (clear_processed_siginfo && sig_state->sig_info[i] != NULL) {
 			uint32_t j;
 			for (j=0;j<count;j++) {
 				uint32_t ofs = (counter.seen + j)