avcodec/mlpdec: Do not leave a invalid num_primitive_matrices in the context

Fixes: runtime error: index 8 out of bounds for type 'uint8_t [8]'
Fixes: 1699/clusterfuzz-testcase-minimized-6327177438035968

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