Fixes: signed integer overflow: 538976288 - -9223372036315799520 cannot be represented in type 'long'
Fixes: 68060/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-5523457266745344

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