Backport of fix for CVE-2015-0973 to libpng 1.2.

This code hasn't changed very much between 1.2 (klp-dev) and
1.5 / 1.6, except for some rearrangement of checks.

bug: 19499430

Change-Id: Ie4cc650b2a5848776d4c49b52ee78b9a6e7cd929
1 file changed