GMenuExporter: allow NULL bus on _name_vanished
GMenuExporter: allow NULL bus on _name_vanished

GBusNameVanishedCallback is called with a NULL GDBusConnection in the
case that the connection has vanished.  We were doing an assert to
verify that it was the same as we had exported the menu on and that
assert was failing.

https://bugzilla.gnome.org/show_bug.cgi?id=685995

gale320/glib,endlessm/glib,ieei/glib,johne53/MB3Glib,endlessm/glib,gale320/glib,lukasz-skalski/glib,cosimoc/glib,gale320/glib,tchakabam/glib,tamaskenez/glib,cosimoc/glib,krichter722/glib,cention-sany/glib,Distrotech/glib,endlessm/glib,ieei/glib,johne53/MB3Glib,tamaskenez/glib,cosimoc/glib,johne53/MB3Glib,MathieuDuponchelle/glib,tchakabam/glib,johne53/MB3Glib,krichter722/glib,MathieuDuponchelle/glib,cention-sany/glib,tamaskenez/glib,djdeath/glib,MathieuDuponchelle/glib,krichter722/glib,tchakabam/glib,krichter722/glib,Distrotech/glib,gale320/glib,endlessm/glib,mzabaluev/glib,cosimoc/glib,djdeath/glib,johne53/MB3Glib,ieei/glib,MathieuDuponchelle/glib,Distrotech/glib,lukasz-skalski/glib,ieei/glib,mzabaluev/glib,lukasz-skalski/glib,Distrotech/glib,tamaskenez/glib,djdeath/glib,Distrotech/glib,djdeath/glib,cosimoc/glib,tchakabam/glib,lukasz-skalski/glib,mzabaluev/glib,MathieuDuponchelle/glib,mzabaluev/glib,tchakabam/glib,cention-sany/glib,cention-sany/glib,gale320/glib,krichter722/glib,ieei/glib,mzabaluev/glib,lukasz-skalski/glib,djdeath/glib,cention-sany/glib,johne53/MB3Glib,endlessm/glib,tamaskenez/glib