IMSG_ERR needs a playlist_dropcurrent(), but otherwise is just the same as IMSG_EOF: the current song ended and we need to move forward. To unify, set current_song to NULL in playlist_dropcurrent, which make sense since right after it we're not playing anything.

this avoids the player skipping two songs on failure.

playlist.songs[play_off] is not a good representation for the current playing song after all. After a `load' command, or a flush request, the play_off changes but we're still playing the "old" song. This saves the current song in a dedicated variable that's updated upon playlist_advance and playlist_previous, thus givin a more accurate view.

The check is useless, the wraparound is already handled in the following line.