Fixed the queue filling up with sentinel events when the WaitEvent call is passed NULL for the event

The use case is an application that waits for events on the main thread and dispatches them on a separate thread.

src/events/SDL_events.c

@@ -891,7 +891,11 @@ SDL_WaitEventTimeout_Device(_THIS, SDL_Window *wakeup_window, SDL_Event * event,
            c) Periodic processing that takes place in some platform PumpEvents() functions happens
            d) Signals received in WaitEventTimeout() are turned into SDL events
         */
-        SDL_PumpEventsInternal(SDL_TRUE);
+        /* We only want a single sentinel in the queue. We could get more than one if event is NULL,
+         * since the SDL_PeepEvents() call below won't remove it in that case.
+         */
+        SDL_bool add_sentinel = (SDL_AtomicGet(&SDL_sentinel_pending) == 0) ? SDL_TRUE : SDL_FALSE;
+        SDL_PumpEventsInternal(add_sentinel);
 
         if (!_this->wakeup_lock || SDL_LockMutex(_this->wakeup_lock) == 0) {
             int status = SDL_PeepEvents(event, 1, SDL_GETEVENT, SDL_FIRSTEVENT, SDL_LASTEVENT);