compositor: make sync_updates_frozen no-op for unmanaged windows

Right now meta_compositor_sync_updates_frozen will crash if it's called
on a window that's not yet managed.

For future convenience, make it be a no-op instead, in the same way:

meta_compositor_window_shape_changed
and
meta_compositor_window_opacity_changed

are no-ops for unmanaged windows.

src/compositor/compositor.c

@@ -684,6 +684,9 @@ meta_compositor_sync_updates_frozen (MetaCompositor *compositor,
                                      MetaWindow     *window)
 {
   MetaWindowActor *window_actor = META_WINDOW_ACTOR (meta_window_get_compositor_private (window));
+  if (!window_actor)
+    return;
+
   meta_window_actor_sync_updates_frozen (window_actor);
 }