wayland: avoid a crash in is_effectively_synchronized()

To check if a subsurface is effectively synchronized, we walk the
subsurface hierarchy to look for a non-subsurface parent or a subsurface
being synchronized.

However, when client is closing, the parent surface might already be
gone, in which case we end up with a surface being NULL which causes a
NULL pointer dereference and a crash.

Check if the parent surface is NULL to avoid the crash, and consider
it's already synchronized if it is NULL to avoid further updates.

Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/124

src/wayland/meta-wayland-surface.c

@@ -573,7 +573,10 @@ meta_wayland_surface_is_effectively_synchronized (MetaWaylandSurface *surface)
         {
           MetaWaylandSurface *parent = surface->sub.parent;
 
+          if (parent)
             return meta_wayland_surface_is_effectively_synchronized (parent);
+
+          return TRUE;
         }
     }
 }