Invalid inserts of items are tested. However, on an invalid insert
of QTreeWidgetItem into QTreeWidget, the QTreeWidget does not take
ownership of the to-be-inserted items. These items were leaked in
the past.
This patch introduces a scope-guard, so the items are not leaked.
Change-Id: Ib53ac02605c3a3c0bd1ae69bbd0876f860d08093
Reviewed-by: Marc Mutz <marc.mutz@qt.io>