avcodec/mjpegdec: Fix integer overflow in DC dequantization

Fixes: runtime error: signed integer overflow: -65535 * 65312 cannot be represented in type 'int'
Fixes: 4900/clusterfuzz-testcase-minimized-5769019744321536

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