avformat/utils: Don't calculate duration using AV_NOPTS_VALUE for start_time.

Found by ClusterFuzz, https://crbug.com/879852Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -3869,7 +3869,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
                 break;
             }
             if (pkt->duration) {
-                if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && pkt->pts >= st->start_time) {
+                if (avctx->codec_type == AVMEDIA_TYPE_SUBTITLE && pkt->pts != AV_NOPTS_VALUE && st->start_time != AV_NOPTS_VALUE && pkt->pts >= st->start_time) {
                     st->info->codec_info_duration = FFMIN(pkt->pts - st->start_time, st->info->codec_info_duration + pkt->duration);
                 } else
                     st->info->codec_info_duration += pkt->duration;