playlist: safely search the playlist for deleted song

When a song file is deleted during database update, all pointers to it
must be removed from the playlist.  The "for" loop in
deleteASongFromPlaylist() did not deal with multiple copies of the
deleted song properly, and left instances of the (to-be-invalidated)
pointer in.  Fix this by reversing the loop.

src/playlist.c

@@ -764,7 +764,7 @@ deleteASongFromPlaylist(const struct song *song)
 	if (NULL == playlist.songs)
 		return;
 
-	for (unsigned i = 0; i < playlist.length; i++)
+	for (int i = playlist.length - 1; i >= 0; --i)
 		if (song == playlist.songs[i])
 			deleteFromPlaylist(i);