core: set source_id if really needed
core: set source_id if really needed

Only insert the source_id in the media if actually the source didn't insert it.

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