wayland/subsurface: Check for parent actor before unparenting

No need to assume the actor still has a parent. This silences warnings
in normal builds and prevents a crash in fully optimized ones.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1958>

src/wayland/meta-wayland-subsurface.c

@@ -303,6 +303,7 @@ unparent_actor (MetaWaylandSurface *surface)
     return;
 
   parent_actor = clutter_actor_get_parent (actor);
+  if (parent_actor)
     clutter_actor_remove_child (parent_actor, actor);
 }