compositor: Prevent an error in application code from keeping unredirect on permanently

We substract one from the unredirect counter when enable_unredirect_for_screen
gets called. It is an unsigned integer so substracting one from zero (which means enable) would overflow and thus keep it peramently enabled.

This should never happen because it means there is an unmatched
enable / disable pair somewhere. So in addition to fixing it add a
warning when this case gets triggered.

https://bugzilla.gnome.org/show_bug.cgi?id=701224