lavf: if timestamps are invalid (pts=dts) only drop dts.

Because if we drop both we have no timestamps at all for some files.
Fixes Ticket344
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -1020,7 +1020,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
     // Note, if this is misbehaving for a H.264 file then possibly presentation_delayed is not set correctly.
     if(delay==1 && pkt->dts == pkt->pts && pkt->dts != AV_NOPTS_VALUE && presentation_delayed){
         av_log(s, AV_LOG_DEBUG, "invalid dts/pts combination %"PRIi64"\n", pkt->dts);
-        pkt->dts= pkt->pts= AV_NOPTS_VALUE;
+        pkt->dts= AV_NOPTS_VALUE;
     }
 
     if (pkt->duration == 0) {