clutter/frame-clock: Warn if frame clock is disposed while dispatching

This shouldn't happen, but warn anyway to be a bit more helpful if
things go bad.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2901>

clutter/clutter/clutter-frame-clock.c

@@ -957,6 +957,8 @@ clutter_frame_clock_dispose (GObject *object)
 {
   ClutterFrameClock *frame_clock = CLUTTER_FRAME_CLOCK (object);
 
+  g_warn_if_fail (frame_clock->state != CLUTTER_FRAME_CLOCK_STATE_DISPATCHING);
+
   if (frame_clock->source)
     {
       g_signal_emit (frame_clock, signals[DESTROY], 0);