avcodec/mjpegdec: Fix runtime error: signed integer overflow: -24543 * 2031616 cannot be represented in type 'int'

Fixes: 943/clusterfuzz-testcase-5114865297391616

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