playlist: let playlist-prev go to last item in playlist

Previously, playlist-prev didn't work if you played a playlist to
completion using --idle and tried to go back. Naturally, one would
expect this to bring up the last item in the playlist, but nothing
happens. This is just because playlist_get_next is stupid and doesn't
take this into account since pl->current is NULL and thus returns NULL.
Fix this by considering the direction, checking if the playlist was
played to completion and grabbing the last entry in the index.