Fix warning with event compression on a destroyed window

Fix a critical message when we try to compress events for a window
that was already destroyed.

gdk/gdkevents.c

@@ -316,6 +316,7 @@ _gdk_event_queue_handle_motion_compression (GdkDisplay *display)
       pending_motions == display->queued_tail)
     {
       GdkFrameClock *clock = gdk_window_get_frame_clock (pending_motion_window);
+      if (clock) /* might be NULL if window was destroyed */
 	gdk_frame_clock_request_phase (clock, GDK_FRAME_CLOCK_PHASE_FLUSH_EVENTS);
     }
 }