playlist/thread: do not stop on removed node
If get_current_status_item( p_playlist ) returns NULL there are no
previously "playing" entity on automatic next, meaning that it is very
confusing if the playlist stops at such (non-existing) entity.
Given a flat playlist, a node containing children will be removed and
the children posted to the node's parent, meaning that the original
node is removed. Given that the node is removed, stopping after
"playback" of such entity looks like we simply did nothing (imagine a
directory containing no children).
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>