avcodec/mjpegdec: Fix negative shift

Fixes: mjpeg_left_shift.avi
Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit d86d7b24)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mjpegdec.c

@@ -975,7 +975,7 @@ static int ljpeg_decode_rgb_scan(MJpegDecodeContext *s, int nb_components, int p
                     return -1;
 
                 left[i] = buffer[mb_x][i] =
-                    mask & (pred + (dc << point_transform));
+                    mask & (pred + (dc * (1 << point_transform)));
             }
 
             if (s->restart_interval && !--s->restart_count) {