Fix warning about frames left in queue.

Example:
- [libmp3lame @ 0x55b9e7248300] 4 frames left in the queue on closing

src/common/ffmpeg.cpp

@@ -450,6 +450,7 @@ void recording::MediaRecorder::Stop()
     npts = 0;
     if (oc)
     {
+        flush_frames();
         // close the output file
         if (!(fmt->flags & AVFMT_NOFILE))
         {
@@ -623,3 +624,16 @@ recording::MediaRet recording::MediaRecorder::AddFrame(const uint16_t *aud, int
     }
     return MRET_OK;
 }
+
+// flush last frames to avoid
+// "X frames left in the queue on closing"
+void recording::MediaRecorder::flush_frames()
+{
+    AVPacket pkt;
+    av_init_packet(&pkt);
+    pkt.data = NULL;
+    pkt.size = 0;
+    // flush last audio frames
+    while (avcodec_receive_packet(aenc, &pkt) >= 0)
+        avcodec_send_frame(aenc, NULL);
+}

src/common/ffmpeg.h

@@ -111,6 +111,9 @@ class MediaRecorder
         MediaRet setup_video_stream(int width, int height);
         MediaRet setup_audio_stream();
         MediaRet finish_setup(const char *fname);
+        // flush last frames to avoid
+        // "X frames left in the queue on closing"
+        void flush_frames();
 };
 
 }