Administration: Ensure that admin referer nonce is valid.

Coding standards, ensure that nonce is valid with identical, rather then equal operator.

Props vortfu, xknown, whyisjake.

Built from https://develop.svn.wordpress.org/trunk@46477


git-svn-id: https://core.svn.wordpress.org/trunk@46275 1a063a9b-81f0-0310-95a4-ce76da25c4cd