yuv4mpeg: set average frame rate

libavformat/yuv4mpeg.c

@@ -361,6 +361,7 @@ static int yuv4_read_header(AVFormatContext *s)
     st->codec->height = height;
     av_reduce(&raten, &rated, raten, rated, (1UL << 31) - 1);
     avpriv_set_pts_info(st, 64, rated, raten);
+    st->avg_frame_rate                = av_inv_q(st->time_base);
     st->codec->pix_fmt                = pix_fmt;
     st->codec->codec_type             = AVMEDIA_TYPE_VIDEO;
     st->codec->codec_id               = AV_CODEC_ID_RAWVIDEO;