ffmpeg: use ist->dts in do_streamcopy()

Based on change by Anton Khirnov.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1902,7 +1902,7 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
         opkt.pts = AV_NOPTS_VALUE;
 
     if (pkt->dts == AV_NOPTS_VALUE)
-        opkt.dts = av_rescale_q(ist->pts, AV_TIME_BASE_Q, ost->st->time_base);
+        opkt.dts = av_rescale_q(ist->dts, AV_TIME_BASE_Q, ost->st->time_base);
     else
         opkt.dts = av_rescale_q(pkt->dts, ist->st->time_base, ost->st->time_base);
     opkt.dts -= ost_tb_start_time;