avformat/utils: Also fill dts==RELATIVE_TS_BASE packets in update_initial_durations()

This dts value can end up in the list in the absence of durations and is in that
case semantically identical to AV_NOPTS_VALUE. We can alternatively prevent
storing RELATIVE_TS_BASE if there is no duration.

Fixes Ticket3640
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -1164,8 +1164,11 @@ static void update_initial_durations(AVFormatContext *s, AVStream *st,
     for (; pktl; pktl = get_next_pkt(s, st, pktl)) {
         if (pktl->pkt.stream_index != stream_index)
             continue;
-        if (pktl->pkt.pts == pktl->pkt.dts  &&
-            (pktl->pkt.dts == AV_NOPTS_VALUE || pktl->pkt.dts == st->first_dts) &&
+        if ((pktl->pkt.pts == pktl->pkt.dts ||
+             pktl->pkt.pts == AV_NOPTS_VALUE) &&
+            (pktl->pkt.dts == AV_NOPTS_VALUE ||
+             pktl->pkt.dts == st->first_dts ||
+             pktl->pkt.dts == RELATIVE_TS_BASE) &&
             !pktl->pkt.duration) {
             pktl->pkt.dts = cur_dts;
             if (!st->internal->avctx->has_b_frames)