gsettings: Don't crash when no schemas are installed
gsettings: Don't crash when no schemas are installed

Still doesn't behave well ("gsettings help" causes an error),
but at least there's no segfault anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=724847

johne53/MB3Glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,Distrotech/glib,endlessm/glib,Distrotech/glib,endlessm/glib,Distrotech/glib,endlessm/glib,endlessm/glib,Distrotech/glib,johne53/MB3Glib,johne53/MB3Glib,MathieuDuponchelle/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,johne53/MB3Glib,Distrotech/glib