Revert "asfenc: return error on negative timestamp"

This reverts commit d1bec33b46091546c5b2e6815210e73f87abf413, it breaks
FATE.

libavformat/asfenc.c

@@ -774,14 +774,6 @@ static int asf_write_packet(AVFormatContext *s, AVPacket *pkt)
         flags &= ~AV_PKT_FLAG_KEY;
 
     pts = (pkt->pts != AV_NOPTS_VALUE) ? pkt->pts : pkt->dts;
-
-    if (pts < 0) {
-        av_log(s, AV_LOG_ERROR,
-               "Negative dts not supported stream %d, dts %"PRId64"\n",
-               pkt->stream_index, pts);
-        return AVERROR(ENOSYS);
-    }
-
     assert(pts != AV_NOPTS_VALUE);
     duration      = pts * 10000;
     asf->duration = FFMAX(asf->duration, duration + pkt->duration * 10000);