image/png: add Fuzz function

Add a Fuzz function to package png, under the gofuzz build
tag. This function is based on the png/png.go code, from
github.com/dvyukov/go-fuzz-corpus, modified to use direct
comparison of image bounds rather than reflect.DeepEqual.

Updates #30979
Updates #19109

Change-Id: Idb86e7ded0c2d78e6cadbeda84c7b1f35b8c579c
Reviewed-on: https://go-review.googlesource.com/c/go/+/168558


Reviewed-by: thepudds <thepudds1460@gmail.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>