Prevent redirect away from wp-login.php when logging out

[userabuser]

Simply by checking whether or not the action "wp_logout" was fired prior to verifying the nonce will prevent the user from being redirected to the result of network_home_url() when logging out.

This PR is in relation to one of the adverse effects listed on the main repository page:

When a user signs out, they are redirected back to the root of the site rather than wp-login.php. This is because the nonce used by the previously signed-in user does not match the nonce of the now signed-out user, Brutus (correctly) detects a mismatch, and bounces the user away from wp-login.php. This could be improved, but I haven't spent enough time trying to unwind the redirection dance here.