ffmpeg: fix vstats

Broken by: 394fb56c29eee7f4f8f0334d8b5d30d3c54ac703
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1396,6 +1396,7 @@ static void flush_encoders(void)
 
             if (encode) {
                 AVPacket pkt;
+                int pkt_size;
                 int got_packet;
                 av_init_packet(&pkt);
                 pkt.data = NULL;
@@ -1426,9 +1427,10 @@ static void flush_encoders(void)
                     pkt.dts = av_rescale_q(pkt.dts, enc->time_base, ost->st->time_base);
                 if (pkt.duration > 0)
                     pkt.duration = av_rescale_q(pkt.duration, enc->time_base, ost->st->time_base);
+                pkt_size = pkt.size;
                 write_frame(os, &pkt, ost);
                 if (ost->st->codec->codec_type == AVMEDIA_TYPE_VIDEO && vstats_filename) {
-                    do_video_stats(ost, pkt.size);
+                    do_video_stats(ost, pkt_size);
                 }
             }