avformat/mov: Fix incorrect overflow detection in mov_read_sidx()

Fixes: signed integer overflow: 9223372036854775807 + 1442840321 cannot be represented in type 'long'
Fixes: 33670/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-6644379491106816

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

libavformat/mov.c

@@ -5098,7 +5098,7 @@ static int mov_read_sidx(MOVContext *c, AVIOContext *pb, MOVAtom atom)
         if (frag_stream_info)
             frag_stream_info->sidx_pts = timestamp;
 
-        if (av_sat_add64(offset, size) != offset + size ||
+        if (av_sat_add64(offset, size) != offset + (uint64_t)size ||
             av_sat_add64(pts, duration) != pts + (uint64_t)duration
         )
             return AVERROR_INVALIDDATA;