avformat/mux: Check for and remove invalid packet durations

Fixes assertion failure
Fixes Ticket3575
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit dc6a17cf)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mux.c

@@ -421,6 +421,12 @@ static int compute_pkt_fields2(AVFormatContext *s, AVStream *st, AVPacket *pkt)
     av_dlog(s, "compute_pkt_fields2: pts:%s dts:%s cur_dts:%s b:%d size:%d st:%d\n",
             av_ts2str(pkt->pts), av_ts2str(pkt->dts), av_ts2str(st->cur_dts), delay, pkt->size, pkt->stream_index);
 
+    if (pkt->duration < 0 && st->codec->codec_type != AVMEDIA_TYPE_SUBTITLE) {
+        av_log(s, AV_LOG_WARNING, "Packet with invalid duration %d in stream %d\n",
+               pkt->duration, pkt->stream_index);
+        pkt->duration = 0;
+    }
+
     /* duration field */
     if (pkt->duration == 0) {
         ff_compute_frame_duration(&num, &den, st, NULL, pkt);