Don't call libvlc_media_list_set_media from media since it causes
p_md->p_subitems to retain p_md while p_md is already retaining
p_md->p_subitems, therefore these 2 objects won't be releasable.

Add a new internal variable: p_internal_md that is set from media without
retaining it.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>