player/Thread: stop buffering if buffer is full

Shouldn't ever happen, but who knows...

src/MusicBuffer.hxx

@@ -29,7 +29,7 @@
  */
 class MusicBuffer {
 	/** a mutex which protects #buffer */
-	Mutex mutex;
+	mutable Mutex mutex;
 
 	SliceBuffer<MusicChunk> buffer;
 
@@ -53,6 +53,11 @@ public:
 	}
 #endif
 
+	bool IsFull() const noexcept {
+		const std::lock_guard<Mutex> protect(mutex);
+		return buffer.IsFull();
+	}
+
 	/**
 	 * Returns the total number of reserved chunks in this buffer.  This
 	 * is the same value which was passed to the constructor