player/Thread: merge four mutex locks in SeekDecoder()

src/decoder/DecoderControl.hxx

@@ -304,12 +304,6 @@ struct DecoderControl {
 	gcc_pure
 	bool IsCurrentSong(const DetachedSong &_song) const noexcept;
 
-	gcc_pure
-	bool LockIsCurrentSong(const DetachedSong &_song) const noexcept {
-		const std::lock_guard<Mutex> protect(mutex);
-		return IsCurrentSong(_song);
-	}
-
 private:
 	/**
 	 * Wait for the command to be finished by the decoder thread.

src/player/Thread.cxx

@@ -615,20 +615,18 @@ Player::SeekDecoder() noexcept
 
 	pc.outputs.Cancel();
 
-	if (!dc.LockIsCurrentSong(*pc.next_song)) {
+	const std::lock_guard<Mutex> lock(pc.mutex);
+
+	if (!dc.IsCurrentSong(*pc.next_song)) {
 		/* the decoder is already decoding the "next" song -
 		   stop it and start the previous song again */
 
-		{
-			const std::lock_guard<Mutex> lock(pc.mutex);
-			StopDecoder();
-		}
+		StopDecoder();
 
 		/* clear music chunks which might still reside in the
 		   pipe */
 		pipe->Clear(buffer);
 
-		const std::lock_guard<Mutex> lock(pc.mutex);
 		/* re-start the decoder */
 		StartDecoder(*pipe);
 		ActivateDecoder();
@@ -642,8 +640,6 @@ Player::SeekDecoder() noexcept
 			ClearAndReplacePipe(dc.pipe);
 		}
 
-		const std::lock_guard<Mutex> lock(pc.mutex);
-
 		pc.next_song.reset();
 		queued = false;
 
@@ -660,7 +656,7 @@ Player::SeekDecoder() noexcept
 			return false;
 	}
 
-	pc.LockCommandFinished();
+	pc.CommandFinished();
 
 	assert(xfade_state == CrossFadeState::UNKNOWN);