wayland: Don't crash when trying to fullscreen on inert wl_output

Merged Jonas Ådahl requested to merge jadahl/mutter:wip/dont-fullscreen-on-inert-output into master

There is a race where an output can be used as a fullscreen target, but it has already been removed due to a hotplug. Handle this gracefully by ignoring said output in such situations.

Merge request reports