lua-factory: Fix possible memleak if source registration fails
lua-factory: Fix possible memleak if source registration fails

Spotted on #grilo

https://bugzilla.gnome.org/show_bug.cgi?id=751786

grilofw/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins