ffmpeg.c: fix erroneous "Output file is empty" message.

This message is printed whenever the RAWVIDEO codec is used, whether the
output file contains any frames or not. Test command:
./ffmpeg -i infile.y4m outfile.y4m

Signed-off-by: Mike Scheutzow <mjs973@optonline.net>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1746,6 +1746,7 @@ duplicate_frame:
         pkt.flags |= AV_PKT_FLAG_KEY;
 
         write_frame(s, &pkt, ost);
+        video_size += pkt.size;
     } else {
         int got_packet;
         AVFrame big_picture;