Fix/work around GitHub bug log4cplus#155.

Here we check that we can get CRT's heap handle because if we do not
then the following `delete` will fail with access violation in
`RtlFreeHeap()`.

How is it possible that the CRT heap handle is NULL?

This function can be called from TLS initializer/terminator by loader
when log4cplus is compiled and linked to as a static library. In case of
other threads temination, it should do its job and free per-thread
data. However, when the whole process is being terminated, it is called
after the CRT has been uninitialized and the CRT heap is not available
any more. In such case, instead of crashing, we just give up and leak
the memory for the short while before the process terminates anyway.

It is possible to work around this situation in user application by
calling `threadCleanup()` manually before `main()` exits.

src/global-init.cxx

@@ -387,9 +387,38 @@ initialize ()
 void
 threadCleanup ()
 {
-    // Do thread-specific cleanup.
-    internal::per_thread_data * ptd = internal::get_ptd (false);
-    delete ptd;
+    // Here we check that we can get CRT's heap handle because if we do not
+    // then the following `delete` will fail with access violation in
+    // `RtlFreeHeap()`.
+    //
+    // How is it possible that the CRT heap handle is NULL?
+    //
+    // This function can be called from TLS initializer/terminator by loader
+    // when log4cplus is compiled and linked to as a static library. In case of
+    // other threads temination, it should do its job and free per-thread
+    // data. However, when the whole process is being terminated, it is called
+    // after the CRT has been uninitialized and the CRT heap is not available
+    // any more. In such case, instead of crashing, we just give up and leak
+    // the memory for the short while before the process terminates anyway.
+    //
+    // It is possible to work around this situation in user application by
+    // calling `threadCleanup()` manually before `main()` exits.
+#if defined (_WIN32)
+    if (_get_heap_handle() != 0)
+    {
+#endif
+        // Do thread-specific cleanup.
+        internal::per_thread_data * ptd = internal::get_ptd (false);
+        delete ptd;
+#if defined (_WIN32)
+    }
+    else
+    {
+        OutputDebugString (
+            LOG4CPLUS_TEXT ("log4cplus: ")
+            LOG4CPLUS_TEXT ("CRT heap is already gone in threadCleanup()\n"));
+    }
+#endif
     internal::set_ptd (0);
 }
 
@@ -463,6 +492,25 @@ thread_callback (LPVOID /*hinstDLL*/, DWORD fdwReason, LPVOID /*lpReserved*/)
     } // switch
 }
 
+
+static
+void NTAPI
+thread_callback_initializer(LPVOID hinstDLL, DWORD fdwReason, LPVOID lpReserved)
+{
+    if (fdwReason == DLL_PROCESS_ATTACH
+        || fdwReason == DLL_THREAD_ATTACH)
+        thread_callback(hinstDLL, fdwReason, lpReserved);
+}
+
+static
+void NTAPI
+thread_callback_terminator(LPVOID hinstDLL, DWORD fdwReason, LPVOID lpReserved)
+{
+    if (fdwReason == DLL_THREAD_DETACH
+        || fdwReason == DLL_PROCESS_DETACH)
+        thread_callback(hinstDLL, fdwReason, lpReserved);
+}
+
 #endif
 
 
@@ -489,22 +537,36 @@ extern "C"
 // This magic has been pieced together from several sources:
 // - <http://www.nynaeve.net/?p=183>
 // - <http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-November/018818.html>
+// - `internal_shared.h` in CRT source in Visual Studio 2015
+
+#pragma data_seg (push, old_seg)
+#ifdef _WIN64
+#pragma const_seg (".CRT$XLY")
+extern const
+#else
+#pragma data_seg (".CRT$XLY")
+#endif
+PIMAGE_TLS_CALLBACK log4cplus_p_thread_callback_initializer = log4cplus::thread_callback_initializer;
+#pragma data_seg (pop, old_seg)
 
 #pragma data_seg (push, old_seg)
 #ifdef _WIN64
-#pragma const_seg (".CRT$XLX")
+#pragma const_seg (".CRT$XLAA")
 extern const
 #else
-#pragma data_seg (".CRT$XLX")
+#pragma data_seg (".CRT$XLAA")
 #endif
-PIMAGE_TLS_CALLBACK log4cplus_p_thread_callback = log4cplus::thread_callback;
+PIMAGE_TLS_CALLBACK log4cplus_p_thread_callback_terminator = log4cplus::thread_callback_terminator;
 #pragma data_seg (pop, old_seg)
+
 #ifdef _WIN64
 #pragma comment (linker, "/INCLUDE:_tls_used")
-#pragma comment (linker, "/INCLUDE:log4cplus_p_thread_callback")
+#pragma comment (linker, "/INCLUDE:log4cplus_p_thread_callback_initializer")
+#pragma comment (linker, "/INCLUDE:log4cplus_p_thread_callback_terminator")
 #else
 #pragma comment (linker, "/INCLUDE:__tls_used")
-#pragma comment (linker, "/INCLUDE:_log4cplus_p_thread_callback")
+#pragma comment (linker, "/INCLUDE:_log4cplus_p_thread_callback_initializer")
+#pragma comment (linker, "/INCLUDE:_log4cplus_p_thread_callback_terminator")
 #endif
 
 } // extern "C"