Quartz target shouldn't crash if an event thread is used.

 (SDL_INIT_EVENTTHREAD still doesn't work, but the crash is gone...)

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401216

src/video/quartz/SDL_QuartzEvents.m

@@ -709,6 +709,9 @@ void QZ_PumpEvents (_THIS)
     NSRect winRect;
     NSAutoreleasePool *pool;
 
+    if (!SDL_VideoSurface)
+        return;  /* don't do anything if there's no screen surface. */
+
     /* Update activity every five seconds to prevent screensaver. --ryan. */
     static Uint32 screensaverTicks = 0;
     Uint32 nowTicks = SDL_GetTicks();