asf: use packet dts as approximation of pts

Having a somehow off seeking is better than having none at all.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/asfdec.c

@@ -1173,7 +1173,7 @@ static int64_t asf_read_pts(AVFormatContext *s, int stream_index, int64_t *ppos,
             return AV_NOPTS_VALUE;
         }
 
-        pts= pkt->pts;
+        pts = pkt->dts;
 
         av_free_packet(pkt);
         if(pkt->flags&AV_PKT_FLAG_KEY){