Use `elseif` when slurping the `nonce` in `check_ajax_referer()` to avoid accidentally overwriting it. 

Fail wonderboymusic in [25433].
Props ocean90.
Fixes #25369.
See [25433].



git-svn-id: https://develop.svn.wordpress.org/trunk@25550 602fd350-edb4-49c9-b593-d223f7449a82