MD5 file verification: Prevent md5_file() warnings when files don't exist, additionally, don't verify wp-content files as they can be updated separately, as well as WP_CONTENT_DIR being set elsewhere. See #22704 See #18201


git-svn-id: https://develop.svn.wordpress.org/trunk@25648 602fd350-edb4-49c9-b593-d223f7449a82