avformat/sbgdec: Check for overflow in timestamp preparation

Fixes: signed integer overflow: 9223372036854775807 + 86400000000 cannot be represented in type 'long'
Fixes: 29102/clusterfuzz-testcase-minimized-ffmpeg_dem_SBG_fuzzer-6731040263634944

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

libavformat/sbgdec.c

@@ -1292,6 +1292,10 @@ static int generate_intervals(void *log, struct sbg_script *s, int sample_rate,
         ev1 = &s->events[i];
         ev2 = &s->events[(i + 1) % s->nb_events];
         ev1->ts_int   = ev1->ts;
+
+        if (!ev1->fade.slide && ev1 >= ev2 && ev2->ts > INT64_MAX - period)
+            return AVERROR_INVALIDDATA;
+
         ev1->ts_trans = ev1->fade.slide ? ev1->ts
                                         : ev2->ts + (ev1 < ev2 ? 0 : period);
     }