only use ldap user backend when it is configured

apps/user_ldap/user_ldap.php

@@ -97,6 +97,9 @@ class OC_USER_LDAP extends OC_USER_BACKEND {
 		return $entries[0]["dn"];
 	}
 	public function checkPassword( $uid, $password ) {
+		if(!self::$configured){
+			return false;
+		}
 		$dn = $this->getDn( $uid );
 		if( !$dn )
 			return false;
@@ -105,6 +108,9 @@ class OC_USER_LDAP extends OC_USER_BACKEND {
 	}
 
 	public function userExists( $uid ) {
+		if(!self::$configured){
+			return false;
+		}
 		$dn = $this->getDn($uid);
 		return !empty($dn);
 	}