avformat/subtitles: treat negative duration like unknown duration

Fixes a specific srt sample, which has an event with negative duration.
libavcodec will convert an event with negative duration to an ASS event
which will be displayed forever, which is not wanted here.

Treat negative duration always as unknown duration instead, and show it
until the next subtitle event.

libavformat/subtitles.c

@@ -198,7 +198,7 @@ void ff_subtitles_queue_finalize(void *log_ctx, FFDemuxSubtitlesQueue *q)
           q->sort == SUB_SORT_TS_POS ? cmp_pkt_sub_ts_pos
                                      : cmp_pkt_sub_pos_ts);
     for (i = 0; i < q->nb_subs; i++)
-        if (q->subs[i].duration == -1 && i < q->nb_subs - 1)
+        if (q->subs[i].duration < 0 && i < q->nb_subs - 1)
             q->subs[i].duration = q->subs[i + 1].pts - q->subs[i].pts;
 
     if (!q->keep_duplicates)