pngout error XSS fix

git-svn-id: https://plugins.svn.wordpress.org/ewww-image-optimizer/trunk@993956 b8457f37-d9ea-0310-8a92-e5e31aec5664
forgot to remove error override for testing previous fix

git-svn-id: https://plugins.svn.wordpress.org/ewww-image-optimizer/trunk@993958 b8457f37-d9ea-0310-8a92-e5e31aec5664