player/Thread: never reuse decoder when switching radio streams

When switching to another song manually, the player checks if the
decoder is already decoding that song; if so, it will attempt to reuse
it by seeking it to the new position.  That however fails if the
decoder is not seekable (e.g. a radio stream) which leaves the user
unable to switch to that song with the bogus error message "Not
seekable".

src/player/Thread.cxx

@@ -584,7 +584,7 @@ Player::SeekDecoder()
 
 	const SongTime start_time = pc.next_song->GetStartTime();
 
-	if (!dc.LockIsCurrentSong(*pc.next_song)) {
+	if (!dc.LockIsSeeakbleCurrentSong(*pc.next_song)) {
 		/* the decoder is already decoding the "next" song -
 		   stop it and start the previous song again */