Log warning when no uid was found for user

In some incomplete setups (like mine) it can happen that the uid
attribute of users is missing.

To be able to find out that something is wrong, a debug message is now
logged when it has not been found.

apps/user_ldap/group_ldap.php

@@ -283,6 +283,10 @@ class GROUP_LDAP extends BackendUtility implements \OCP\GroupInterface {
 			$uid = $userDN;
 		} else if(strtolower($this->access->connection->ldapGroupMemberAssocAttr) === 'memberuid') {
 			$result = $this->access->readAttribute($userDN, 'uid');
+			if ($result === false) {
+				\OCP\Util::writeLog('user_ldap', 'No uid attribute found for DN ' . $userDN . ' on '.
+					$this->access->connection->ldapHost, \OCP\Util::DEBUG);
+			}
 			$uid = $result[0];
 		} else {
 			// just in case