avformat/sbgdec: Check ts_int in genrate_intervals

There is probably a better place to check for this, but better
here than nowhere

Fixes: signed integer overflow: -9223372036824775808 - 86400000000 cannot be represented in type 'long'
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_SBG_fuzzer-6601162580688896

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

@@ -1317,6 +1317,8 @@ static int generate_intervals(void *log, struct sbg_script *s, int sample_rate,
 
     /* Pseudo event before the first one */
     ev0 = s->events[s->nb_events - 1];
+    if (av_sat_sub64(ev0.ts_int, period) != (uint64_t)ev0.ts_int - period)
+        return AVERROR_INVALIDDATA;
     ev0.ts_int   -= period;
     ev0.ts_trans -= period;
     ev0.ts_next  -= period;