fix dbghlp heap corruption on windows

synchronize access to dbghlp.dll on windows

src/assert.cpp

@@ -147,6 +147,7 @@ TORRENT_EXPORT void print_backtrace(char* out, int len, int max_depth, void*)
 
 #include "windows.h"
 #include "libtorrent/utf8.hpp"
+#include <mutex>
 
 #include "winbase.h"
 #include "dbghelp.h"
@@ -154,6 +155,11 @@ TORRENT_EXPORT void print_backtrace(char* out, int len, int max_depth, void*)
 TORRENT_EXPORT void print_backtrace(char* out, int len, int max_depth
 	, void* ctx)
 {
+	// all calls to DbgHlp.dll are thread-unsafe. i.e. they all need to be
+	// synchronized and not called concurrently. This mutex serializes access
+	static std::mutex dbghlp_mutex;
+	std::lock_guard<std::mutex> l(dbghlp_mutex);
+
 	CONTEXT context_record;
 	if (ctx)
 	{

test/main.cpp

@@ -52,6 +52,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <windows.h> // fot SetErrorMode
 #include <io.h> // for _dup and _dup2
 #include <process.h> // for _getpid
+#include <crtdbg.h>
 
 #define dup _dup
 #define dup2 _dup2
@@ -279,6 +280,11 @@ EXPORT int main(int argc, char const* argv[])
 
 	SetUnhandledExceptionFilter(&seh_exception_handler);
 
+#ifdef _DEBUG
+	_CrtSetReportMode(_CRT_ERROR, _CRTDBG_MODE_FILE);
+	_CrtSetReportFile(_CRT_ERROR, _CRTDBG_FILE_STDERR);
+#endif
+
 #else
 
 	signal(SIGSEGV, &sig_handler);