avcodec/mimic: Fix runtime error: index 96 out of bounds for type 'const int8_t [64]'

Fixes: 1468/clusterfuzz-testcase-minimized-5235964056174592

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
