gthread: Emit a critical if g_rw_lock_reader_lock() fails

It can only fail if there’s been a leak or programmer error, so this is
really unlikely to happen. At least make it obvious something has gone
wrong, though, rather than silently carrying on and returning as if the
reader lock has been acquired.

Do the same for g_rw_lock_writer_lock().

It should be safe to use g_critical() for reporting the problems, since
GRWLock is not used in gmessages.c, and printing a critical seems better
than aborting, just in case we do hit the ‘maximum number of reader
locks’ error code.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=756430