barrier: Complain loudly if bogus barriers are created

The API allows for invalid barriers to be created; in an X11 session,
this could result in involutary early exit, so guard against those with
soft asserts. These will be logged in the journal as warnings, but will
avoid the crash unless compiled out.

Note that this doesn't fix the bug, it just makes it more detectable.

Related: https://bugzilla.redhat.com/show_bug.cgi?id=1901610


(cherry picked from commit f5949af0)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1613>