decoder/Control: caller must lock mutex for Stop()

src/decoder/DecoderControl.cxx

@@ -112,8 +112,6 @@ DecoderControl::Start(std::unique_ptr<DetachedSong> _song,
 void
 DecoderControl::Stop() noexcept
 {
-	const std::lock_guard<Mutex> protect(mutex);
-
 	if (command != DecoderCommand::NONE)
 		/* Attempt to cancel the current command.  If it's too
 		   late and the decoder thread is already executing

src/decoder/DecoderControl.hxx

@@ -383,6 +383,9 @@ public:
 		   SongTime start_time, SongTime end_time,
 		   MusicBuffer &buffer, MusicPipe &pipe) noexcept;
 
+	/**
+	 * Caller must lock the object.
+	 */
 	void Stop() noexcept;
 
 	/**

src/player/Thread.cxx

@@ -373,6 +373,7 @@ void
 Player::StopDecoder() noexcept
 {
 	const PlayerControl::ScopeOccupied occupied(pc);
+	const std::lock_guard<Mutex> protect(pc.mutex);
 
 	dc.Stop();