Move the Null-Byte LDAP check to the user manager

The existing method is deprecated and just a wrapper around the manager method. Since in the future other code paths might call this function instead we need to perform that check here.

Related to http://owncloud.org/security/advisory/?id=oc-sa-2014-020
Merge pull request #12901 from owncloud/move-ldap-check-to-manager

Move the Null-Byte LDAP check to the user manager