Check for non-zero value passed to g_flags_get_first_value()

When the value is zero, the flag removal does not do anything, which
leads to an infinite loop.

It is related to a GLib change:
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2576

This had been reported downstream:
https://bugzilla.redhat.com/show_bug.cgi?id=2107751