Fixes: Use of uninitialized memory
Fixes: 71444/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-5448597561212928
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>