Quietly ignore GDK_NOTHING events

These don't have a surface, so we can't deliver
them via the ::event signal. But then, they're
good for nothing anyway.

gdk/gdkevents.c

@@ -149,6 +149,9 @@ gdk_event_class_init (GdkEventClass *klass)
 void
 _gdk_event_emit (GdkEvent *event)
 {
+  if (gdk_event_get_event_type (event) == GDK_NOTHING)
+    return;
+
   if (gdk_drag_handle_source_event (event))
     return;