image/png: fix crash when an alleged PNG has too much pixel data, so that the zlib.Reader returns nil error.
image/png: fix crash when an alleged PNG has too much pixel data,
so that the zlib.Reader returns nil error.

Fixes issue 7762.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/86750044

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old