avformat/asfdec_o: Check size vs. offset in detect_unknown_subobject()

Fixes: signed integer overflow: 2314885530818453566 + 7503032301549264928 cannot be represented in type 'long'
Fixes: 26639/clusterfuzz-testcase-minimized-ffmpeg_dem_ASF_O_fuzzer-6024222100684800

Alternatively this could be ignored but then the end condition of the loop
would be hard to reach as avio_tell() is int64_t

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 0bee216ad4)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/asfdec_o.c

@@ -1679,6 +1679,9 @@ static int detect_unknown_subobject(AVFormatContext *s, int64_t offset, int64_t
     ff_asf_guid guid;
     int ret;
 
+    if (offset > INT64_MAX - size)
+        return AVERROR_INVALIDDATA;
+
     while (avio_tell(pb) <= offset + size) {
         if (avio_tell(pb) == asf->offset)
             break;