sharing: Prevent an endless loop and resulting stack overflow

sharing: Prevent an endless loop and resulting stack overflow

We need to block ourselves around the gtk_switch_set_active() call in
the error path, when we can't speak to the sharing dbus service, or we
end up in an endless loop since we're the ::state-set handler.

https://bugzilla.gnome.org/show_bug.cgi?id=739886

Gbp-Pq: Name 30_stack_overflow.patch