surface-actor: Add NULL check for stage
In some cases Wayland surface actors are not decendents of the stage, notably subsurfaces without a parent surface. The code changes in the mentioned commits dropped the previously existing check for that - readd it.
Fixes: 627ab247 (surface-actor: Add meta_surface_actor_schedule_update()) Fixes: 7eb99dde (wayland/surface: Schedule updates in the context of the surface actor)
Closes: #3323 (closed)