If a client asks for xdg-output before we have set the output's logical
monitor, we would end up crashing with a NULL pointer dereference.
Make sure we clear the resource's user data when marking an output as
inert on monitor change so that we don't end up with a Wayland output
without a logical monitor.
Closes: https://gitlab.gnome.org/GNOME/mutter/issues/194
(cherry picked from commit 48eaa36d41bb88d4831e40e9c3ef3c7afda195bc)
f45af3315d