mod_auth_form: revert r1585431 (don't crash when no AuthFormProvider is configured).

This cannot happen in trunk, 2.4.x needs authenticate_form_post_config() to be merged.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1585434 13f79535-47bb-0310-9956-ffa450edef68