lavf: Don't drop both pts and dts if timestamps are invalid

In these cases, only drop dts. Because if we drop both we have no
timestamps at all for some files.

This improves playback of HLS streams from GoPro cameras.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/utils.c

@@ -616,7 +616,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
     if (delay == 1 && pkt->dts == pkt->pts &&
         pkt->dts != AV_NOPTS_VALUE && presentation_delayed) {
         av_log(s, AV_LOG_DEBUG, "invalid dts/pts combination\n");
-        pkt->dts = pkt->pts = AV_NOPTS_VALUE;
+        pkt->dts = AV_NOPTS_VALUE;
     }
 
     if (pkt->duration == 0 && st->codec->codec_type != AVMEDIA_TYPE_AUDIO) {