avcodec/v4l2_m2m_enc: buffer frame if it cannot be enqueued

Currently if the frame buffers are full, the frame is unrefed and
dropped.  Instead buffer the frame so that it is enqueued in the
next v4l2_receive_packet() call.  The behavior was observed on
DragonBoard 410c.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>

libavcodec/v4l2_m2m.c

@@ -329,6 +329,7 @@ static void v4l2_m2m_destroy_context(void *opaque, uint8_t *context)
     sem_destroy(&s->refsync);
 
     close(s->fd);
+    av_frame_unref(s->frame);
     av_frame_free(&s->frame);
     av_packet_unref(&s->buf_pkt);
 

libavcodec/v4l2_m2m_enc.c

@@ -295,16 +295,20 @@ static int v4l2_receive_packet(AVCodecContext *avctx, AVPacket *avpkt)
     if (s->draining)
         goto dequeue;
 
+    if (!frame->buf[0]) {
     ret = ff_encode_get_frame(avctx, frame);
     if (ret < 0 && ret != AVERROR_EOF)
         return ret;
 
     if (ret == AVERROR_EOF)
         frame = NULL;
+    }
 
     ret = v4l2_send_frame(avctx, frame);
-    av_frame_unref(frame);
-    if (ret < 0)
+    if (ret != AVERROR(EAGAIN))
+        av_frame_unref(frame);
+
+    if (ret < 0 && ret != AVERROR(EAGAIN))
         return ret;
 
     if (!output->streamon) {