avformat/utils: Do not assume duration is non negative in compute_pkt_fields()

Several subtitle demuxers set negative durations

Fixes: signed integer overflow: 9223372036854775807 - -1 cannot be represented in type 'long'
Fixes: 16925/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5766519790764032

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -1299,7 +1299,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
     }
 
     duration = av_mul_q((AVRational) {pkt->duration, 1}, st->time_base);
-    if (pkt->duration == 0) {
+    if (pkt->duration <= 0) {
         ff_compute_frame_duration(s, &num, &den, st, pc, pkt);
         if (den && num) {
             duration = (AVRational) {num, den};
@@ -1310,7 +1310,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
         }
     }
 
-    if (pkt->duration != 0 && (s->internal->packet_buffer || s->internal->parse_queue))
+    if (pkt->duration > 0 && (s->internal->packet_buffer || s->internal->parse_queue))
         update_initial_durations(s, st, pkt->stream_index, pkt->duration);
 
     /* Correct timestamps with byte offset if demuxers only have timestamps
@@ -1370,7 +1370,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
              * by knowing the future. */
         } else if (pkt->pts != AV_NOPTS_VALUE ||
                    pkt->dts != AV_NOPTS_VALUE ||
-                   pkt->duration                ) {
+                   pkt->duration > 0             ) {
 
             /* presentation is not delayed : PTS and DTS are the same */
             if (pkt->pts == AV_NOPTS_VALUE)
@@ -1380,7 +1380,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
             if (pkt->pts == AV_NOPTS_VALUE)
                 pkt->pts = st->cur_dts;
             pkt->dts = pkt->pts;
-            if (pkt->pts != AV_NOPTS_VALUE)
+            if (pkt->pts != AV_NOPTS_VALUE && duration.num >= 0)
                 st->cur_dts = av_add_stable(st->time_base, pkt->pts, duration, 1);
         }
     }