Clean up locking in g_object_notify_queue_add
Clean up locking in g_object_notify_queue_add

Instaed of returning with the lock held, simply assert
that this cannot happen:

https://bugzilla.gnome.org/show_bug.cgi?id=749678#c4

tchakabam/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,ieei/glib,cention-sany/glib,MathieuDuponchelle/glib,ieei/glib,Distrotech/glib,johne53/MB3Glib,MathieuDuponchelle/glib,johne53/MB3Glib,Distrotech/glib,Distrotech/glib,johne53/MB3Glib,mzabaluev/glib,cention-sany/glib,mzabaluev/glib,tchakabam/glib,mzabaluev/glib,endlessm/glib,Distrotech/glib,endlessm/glib,mzabaluev/glib,johne53/MB3Glib,tchakabam/glib,Distrotech/glib,endlessm/glib,cention-sany/glib,MathieuDuponchelle/glib,endlessm/glib,tchakabam/glib,cention-sany/glib,mzabaluev/glib,cention-sany/glib,endlessm/glib,johne53/MB3Glib,ieei/glib,ieei/glib,ieei/glib,tchakabam/glib,johne53/MB3Glib