avcodec/mjpegdec: Clip DC also on the negative side.

Fixes: runtime error: signed integer overflow: -16711425 + -2130772346 cannot be represented in type 'int'
Fixes: 2533/clusterfuzz-testcase-minimized-5372857678823424

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