Before doing any bind check that the provided username is not NULL and that the

Before doing any bind check that the provided username is not NULL and that the
password is neither NULL nor empty.

Binds with empty passwords always succeed, but in case the password of the
user was not empty subsequent LDAP operations fail.
This causes authentications that use user supplied credentials
(AuthLDAPInitialBindAsUser set to on) to fail with status code 500 instead of
401 if the user supplied an empty password.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1885940 13f79535-47bb-0310-9956-ffa450edef68