Fixed bug #144

Make sure we don't crash or deadlock if someone tries to free a timer after
the timer subsystem has been shut down.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401747

src/timer/SDL_timer.c

@@ -95,6 +95,7 @@ void SDL_TimerQuit(void)
 	}
 	if ( SDL_timer_threaded ) {
 		SDL_DestroyMutex(SDL_timer_mutex);
+		SDL_timer_mutex = NULL;
 	}
 	SDL_timer_started = 0;
 	SDL_timer_threaded = 0;