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. (aka "only relevant for master")

Related to http://owncloud.org/security/advisory/?id=oc-sa-2014-020

@blizzz @icewind1991 Please review.