Fix yet another shift of negative value.
Fix yet another shift of negative value.

dloeng/guetzli,szabadka/guetzli,google/guetzli,google/guetzli,ianhuang-777/guetzli,google/guetzli,ianhuang-777/guetzli,guanzidashu/guetzli_ios_metal,guanzidashu/guetzli_ios_metal,szabadka/guetzli,dloeng/guetzli,dloeng/guetzli,google/guetzli,guanzidashu/guetzli_ios_metal,ianhuang-777/guetzli,guanzidashu/guetzli_ios_metal,szabadka/guetzli,ianhuang-777/guetzli