player/Thread: merge two mutex locks

src/player/Thread.cxx

@@ -610,6 +610,8 @@ Player::SeekDecoder() noexcept
 			ClearAndReplacePipe(dc.pipe);
 		}
 
+		const std::lock_guard<Mutex> lock(pc.mutex);
+
 		const SongTime start_time = pc.next_song->GetStartTime();
 		pc.next_song.reset();
 		queued = false;
@@ -618,7 +620,7 @@ Player::SeekDecoder() noexcept
 		   (just in case that happens to be still in
 		   progress) */
 
-		if (!LockWaitDecoderStartup())
+		if (!WaitDecoderStartup())
 			return false;
 
 		/* send the SEEK command */
@@ -630,8 +632,6 @@ Player::SeekDecoder() noexcept
 				where = total_time;
 		}
 
-		const std::lock_guard<Mutex> lock(pc.mutex);
-
 		try {
 			const PlayerControl::ScopeOccupied occupied(pc);