oavformat/avidec: Check offset in odml

Fixes: signed integer overflow: 9223372036854775807 + 8 cannot be represented in type 'long'
Fixes: 38787/clusterfuzz-testcase-minimized-ffmpeg_dem_AVI_fuzzer-4859845799444480

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

libavformat/avidec.c

@@ -244,7 +244,7 @@ static int read_odml_index(AVFormatContext *s, int64_t frame_num)
             avio_rl32(pb);       /* size */
             duration = avio_rl32(pb);
 
-            if (avio_feof(pb))
+            if (avio_feof(pb) || offset > INT64_MAX - 8)
                 return AVERROR_INVALIDDATA;
 
             pos = avio_tell(pb);