Fixes: Assertion klv_offset >= mxf->run_in failed at libavformat/mxfdec.c:736
Fixes: 62936/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-5778404366221312.fuzz

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>