avformat/mxfdec: Check for duplicate mxf_read_index_entry_array()

Fixes: memleak
Fixes: 41596/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-6439060204290048

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>