lavf/srtenc: allow zero duration events.

This is useful when you want to remux events with zero (or too small)
durations.

libavformat/srtenc.c

@@ -65,7 +65,7 @@ static int srt_write_packet(AVFormatContext *avf, AVPacket *pkt)
         if (d <= 0)
             /* For backward compatibility, fallback to convergence_duration. */
             d = pkt->convergence_duration;
-        if (s == AV_NOPTS_VALUE || d <= 0) {
+        if (s == AV_NOPTS_VALUE || d < 0) {
             av_log(avf, AV_LOG_ERROR, "Insufficient timestamps.\n");
             return AVERROR(EINVAL);
         }