player/Thread: move start_time variable into the scope

src/player/Thread.cxx

@@ -580,8 +580,6 @@ Player::SeekDecoder() noexcept
 
 	pc.outputs.Cancel();
 
-	const SongTime start_time = pc.next_song->GetStartTime();
-
 	if (!dc.LockIsCurrentSong(*pc.next_song)) {
 		/* the decoder is already decoding the "next" song -
 		   stop it and start the previous song again */
@@ -605,6 +603,7 @@ Player::SeekDecoder() noexcept
 			ClearAndReplacePipe(dc.pipe);
 		}
 
+		const SongTime start_time = pc.next_song->GetStartTime();
 		pc.next_song.reset();
 		queued = false;