avformat/sbgdec: Check for overflow in parse_timestamp()

Fixes: signed integer overflow: 33986707200000000 + 9195561788997000192 cannot be represented in type 'long'
Fixes: 23790/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6554232198266880

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

@@ -474,6 +474,8 @@ static int parse_timestamp(struct sbg_parser *p,
     while (lex_char(p, '+')) {
         if (!lex_time(p, &dt))
             return AVERROR_INVALIDDATA;
+        if (av_sat_add64(rel, dt) - dt != rel)
+            return AVERROR_INVALIDDATA;
         rel += dt;
         r = 1;
     }