fix for crash in bug #2991... We still need to figure out why the EventQueue is not being returned properly on the Mac.

xpcom/src/nsEventQueueService.cpp

@@ -230,6 +230,7 @@ nsEventQueueServiceImpl::GetThreadEventQueue(PRThread* aThread, PLEventQueue** a
     *aResult = evQueueEntry->GetEventQueue();
   } else {
     // XXX: Need error code for requesting an event queue when none exists...
+    *aResult = NULL;
     rv = NS_ERROR_FAILURE;
     goto done;
   }

xpcom/threads/nsEventQueueService.cpp

@@ -230,6 +230,7 @@ nsEventQueueServiceImpl::GetThreadEventQueue(PRThread* aThread, PLEventQueue** a
     *aResult = evQueueEntry->GetEventQueue();
   } else {
     // XXX: Need error code for requesting an event queue when none exists...
+    *aResult = NULL;
     rv = NS_ERROR_FAILURE;
     goto done;
   }