wayland/subsurface: Make sure siblings are actually sub-surfaces

Without this check, is_sibling could spuriously return TRUE when neither
surface passed in was actually a sub-surface.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3737>

src/wayland/meta-wayland-subsurface.c

@@ -104,6 +104,7 @@ is_sibling (MetaWaylandSurface *surface,
             MetaWaylandSurface *sibling)
 {
   return surface != sibling &&
+         surface->committed_state.parent &&
          surface->committed_state.parent == sibling->committed_state.parent;
 }