Check the debugger_thread_exited condition in a loop to guard against spurious...

Check the debugger_thread_exited condition in a loop to guard against spurious wakeups. Fixes #642780.

mono/mini/debugger-agent.c

@@ -906,7 +906,7 @@ mono_debugger_agent_cleanup (void)
 	//WaitForSingleObject (debugger_thread_handle, INFINITE);
 	if (GetCurrentThreadId () != debugger_thread_id) {
 		mono_mutex_lock (&debugger_thread_exited_mutex);
-		if (!debugger_thread_exited) {
+		while (!debugger_thread_exited) {
 #ifdef HOST_WIN32
 			if (WAIT_TIMEOUT == WaitForSingleObject(debugger_thread_exited_cond, 0)) {
 				mono_mutex_unlock (&debugger_thread_exited_mutex);