Ignore superfluous trailing IDAT chunks which were not consumed when decoding
the image. This change fixes decoding of valid images in which a zero-length
IDAT chunk appears after the actual image data. It also prevents decoding of
trailing garbage IDAT chunks or maliciously embedded additional images.

Fixes #14936

Change-Id: I8c76cfa9a03496d9576f72bed2db109271f97c5e
Reviewed-on: https://go-review.googlesource.com/21045Reviewed-by: Nigel Tao <nigeltao@golang.org>