avcodec/mjpegdec: Fix runtime error: left shift of negative value -127

Fixes: 733/clusterfuzz-testcase-4682158096515072

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

libavcodec/mjpegdec.c

@@ -1196,13 +1196,13 @@ static int ljpeg_decode_yuv_scan(MJpegDecodeContext *s, int predictor,
                             PREDICT(pred, ptr[-linesize-1], ptr[-linesize], ptr[-1], predictor);
 
                             pred &= mask;
-                            *ptr = pred + (dc << point_transform);
+                            *ptr = pred + ((unsigned)dc << point_transform);
                         }else{
                             ptr16 = (uint16_t*)(s->picture_ptr->data[c] + 2*(linesize * (v * mb_y + y)) + 2*(h * mb_x + x)); //FIXME optimize this crap
                             PREDICT(pred, ptr16[-linesize-1], ptr16[-linesize], ptr16[-1], predictor);
 
                             pred &= mask;
-                            *ptr16= pred + (dc << point_transform);
+                            *ptr16= pred + ((unsigned)dc << point_transform);
                         }
 
                         if (++x == h) {