avformat/mxfdec: More offset_temp checks

avformat/mxfdec: More offset_temp checks

Fixes: signed integer overflow: 9223372036854775807 - -1927491430256034080 cannot be represented in type 'long'
Fixes: 70607/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-5282235077951488

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Tomas Härdin <[email protected]>
Signed-off-by: Michael Niedermayer <[email protected]>
(cherry picked from commit 5a96aa435af0d66bdec52ee115cf4dd971855fcd)
Signed-off-by: Michael Niedermayer <[email protected]>