fix(purge): prevent SQL query with 'true' on WHERE clause

Author: stonebuzz

src/CommonDBTM.php

@@ -887,7 +887,7 @@ public function cleanRelationData()
                 foreach ($fields as $field) {
                     if (is_array($field)) {
                         // Relation based on 'itemtype'/'items_id' (polymorphic relationship)
-                        if ($this instanceof IPAddress && in_array('mainitemtype', $field) && in_array('mainitems_id', $field)) {
+                        if ($itemtype instanceof IPAddress && in_array('mainitemtype', $field) && in_array('mainitems_id', $field)) {
                             // glpi_ipaddresses relationship that does not respect naming conventions
                             $itemtype_field = 'mainitemtype';
                             $items_id_field = 'mainitems_id';