core: Fix memory leak in get_additional_sources
core: Fix memory leak in get_additional_sources

missing_in_data creates a new list which was not being freed by
get_additional_sources.

https://bugzilla.gnome.org/show_bug.cgi?id=744408

MathieuDuponchelle/grilo,MathieuDuponchelle/grilo,GNOME/grilo,grilofw/grilo,jasuarez/grilo,grilofw/grilo,jasuarez/grilo,MathieuDuponchelle/grilo,jasuarez/grilo,GNOME/grilo,grilofw/grilo