Fix undefined variables warnings. Ref: http://wordpress.org/support/topic/plugin-wp-super-cache-lot-of-undefined-variables-and-couple-of-warnings?replies=4


git-svn-id: https://plugins.svn.wordpress.org/wp-super-cache/trunk@873552 b8457f37-d9ea-0310-8a92-e5e31aec5664