[release-branch.go1.8] image/png: restore Go 1.7 rejection of transparent gray8 images

[release-branch.go1.8] image/png: restore Go 1.7 rejection of transparent gray8 images

Go 1.7 and earlier rejected these images with chunkOrderError.
Go 1.8 panicked during decoding.
Go 1.9 will handle them successfully.

Make Go 1.8.1 match Go 1.7 and earlier, to remove the panic
without introducing new functionality in a minor release.

Fixes #19553.

Change-Id: I3c73a27aa3932300326273b6b563cdf606f3ab64
Reviewed-on: https://go-review.googlesource.com/39593
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>