image/png: reject zero-width and zero-height images.

image/png: reject zero-width and zero-height images.

http://www.w3.org/TR/PNG/#11IHDR says that "Zero is an invalid value".

This change only affects the decoder. The encoder already checks
non-positive instead of negative.

Fixes #12545.

Change-Id: Iba40e1a2f4e0eec8b2fbcd3bbdae886311434da7
Reviewed-on: https://go-review.googlesource.com/14411
Reviewed-by: Rob Pike <r@golang.org>