avcodec/mjpegdec: quant_matrixes can be up to 65535, use uint16_t

Fixes invalid shift
Fixes: 870/clusterfuzz-testcase-5649105424482304

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