Fixes: out of array write
Fixes: 32116/clusterfuzz-testcase-minimized-ffmpeg_dem_SIMBIOSIS_IMX_fuzzer-6702533894602752
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>