window: Guard can_ping() against unmanaging windows

We remove pending pings when unmanaging a window, but currently
don't prevent new pings to be scheduled after that.

The previous commit fixed a code path where this did indeed happen,
but as the result of gnome-shell trying to attach a Clutter actor
to a non-existent window actor is pretty bad, also guard can_ping()
against being called for an unmanaging window.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2467

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1676>
(cherry picked from commit 81f36948)