Add missing av_free_packet.

Fixes memory leak when encoding at least with mpegvideo using the
new encode2 function.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

ffmpeg.c

@@ -1633,6 +1633,7 @@ static void do_video_out(AVFormatContext *s, OutputStream *ost,
                 write_frame(s, &pkt, ost);
                 frame_size = pkt.size;
                 video_size += pkt.size;
+                av_free_packet(&pkt);
 
                 /* if two pass, output log */
                 if (ost->logfile && enc->stats_out) {