avcodec/mlpdec: Do not leave invalid values in matrix_out_ch[] on error

Fixes: runtime error: index 12 out of bounds for type 'uint8_t [8]'
Fixes: 1832/clusterfuzz-testcase-minimized-6574546079449088

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