avformat/sbgdec: Check for timestamp overflow in parse_time_sequence()

Fixes: signed integer overflow: 3458015007900000256 + 6425686373040000000 cannot be represented in type 'long'
Fixes: 26430/clusterfuzz-testcase-minimized-ffmpeg_dem_BRSTM_fuzzer-5761175004119040

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

libavformat/sbgdec.c

@@ -538,6 +538,9 @@ static int parse_time_sequence(struct sbg_parser *p, int inblock)
         return AVERROR_INVALIDDATA;
     }
     ts.type = p->current_time.type;
+
+    if (av_sat_add64(p->current_time.t, rel_ts) != p->current_time.t + (uint64_t)rel_ts)
+        return AVERROR_INVALIDDATA;
     ts.t    = p->current_time.t + rel_ts;
     r = parse_fade(p, &fade);
     if (r < 0)