avformat/utils: do not override pts in h264 when they are provided from the demuxer

Fixes Ticket2143
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -1098,12 +1098,14 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
             if (pkt->dts != AV_NOPTS_VALUE) {
                 // got DTS from the stream, update reference timestamp
                 st->reference_dts = pkt->dts - pc->dts_ref_dts_delta * num / den;
-                pkt->pts = pkt->dts + pc->pts_dts_delta * num / den;
             } else if (st->reference_dts != AV_NOPTS_VALUE) {
                 // compute DTS based on reference timestamp
                 pkt->dts = st->reference_dts + pc->dts_ref_dts_delta * num / den;
-                pkt->pts = pkt->dts + pc->pts_dts_delta * num / den;
             }
+
+            if (st->reference_dts != AV_NOPTS_VALUE && pkt->pts == AV_NOPTS_VALUE)
+                pkt->pts = pkt->dts + pc->pts_dts_delta * num / den;
+
             if (pc->dts_sync_point > 0)
                 st->reference_dts = pkt->dts; // new reference
         }