playlist: Do not update score if no playlist is open.

This bug caused the score of the current audio file to be updated
twice in mood mode since both the mood event handler and playlist
event handler honor afs events and update the score.

We fix the bug by moving the check whether a playlist is open to the
top of the playlist event handler, similar to the check in the mood
event handler of mood.c. If no playlist is open, we now return early
from the playlist event handler, ignoring all afs events.