jchuff.c: Clean up aadd60ae + add code comments Based on: https://github.com/fhanau/libjpeg-turbo/commit/4d73f52032a608262f8b286dd4584836ddc73180 https://github.com/fhanau/libjpeg-turbo/commit/184f10703eeb214177b02f88939af237267a8d66 Refer to #602