os: add invalidate_all packets to Audio_out

When a stream was stopped it may still contain valid packets that
will be played automatically when the stream is started again. The
invalidate_all() method may be called after stopping the stream to
prevent this.

Issue #1647.

repos/os/include/audio_out_session/audio_out_session.h

@@ -244,6 +244,16 @@ class Audio_out::Stream
 		void reset() { _tail = _pos; }
 
 
+		/**
+		 * Invalidate all packets in stream queue
+		 */
+		void invalidate_all()
+		{
+			for (int i = 0; i < QUEUE_SIZE; i++)
+				_buf[i]._valid = false;
+		}
+
+
 		/**********************************************
 		 ** Intended to be called by the server side **
 		 ***********************************************/