task: avoid context lock when setting source name
task: avoid context lock when setting source name

If you set the source name after attaching to the context, you have to
lock the context to free/assign the new source name.

https://bugzilla.gnome.org/show_bug.cgi?id=765861

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