window-actor: Do not request unredirection when destroyed

WindowActors can outlive their corresponding window to animate unmap.
Unredirecting the actor does not make sense in that case, so make
sure to not request it.

https://bugzilla.gnome.org/show_bug.cgi?id=740133

src/compositor/meta-window-actor.c

@@ -1155,7 +1155,7 @@ gboolean
 meta_window_actor_should_unredirect (MetaWindowActor *self)
 {
   MetaWindowActorPrivate *priv = self->priv;
-  if (priv->surface)
+  if (!meta_window_actor_is_destroyed (self) && priv->surface)
     return meta_surface_actor_should_unredirect (priv->surface);
   else
     return FALSE;