avienc: Disallow the first frame to be skiped

Fixes Ticket2386
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avienc.c

@@ -523,7 +523,7 @@ static int avi_write_packet(AVFormatContext *s, AVPacket *pkt)
     int size= pkt->size;
 
     av_dlog(s, "dts:%s packet_count:%d stream_index:%d\n", av_ts2str(pkt->dts), avist->packet_count, stream_index);
-    while(enc->block_align==0 && pkt->dts != AV_NOPTS_VALUE && pkt->dts > avist->packet_count && enc->codec_id != AV_CODEC_ID_XSUB){
+    while(enc->block_align==0 && pkt->dts != AV_NOPTS_VALUE && pkt->dts > avist->packet_count && enc->codec_id != AV_CODEC_ID_XSUB && avist->packet_count){
         AVPacket empty_packet;
 
         if(pkt->dts - avist->packet_count > 60000){