Fix ldap connection use after release in password plugin

plugins/password/drivers/ldap.php

@@ -187,12 +187,14 @@ class rcube_ldap_password
         );
 
         $result = $ldap->search($base, $filter, $options);
-        $ldap->done();
         if (is_a($result, 'PEAR_Error') || ($result->count() != 1)) {
+            $ldap->done();
             return '';
         }
+        $userDN = $result->current()->dn();
+        $ldap->done();
 
-        return $result->current()->dn();
+        return $userDN;
     }
 
     /**