TreeModel: Assert when inserting items that already have a model

Makes the assumption Item::m_model != 0 if and only if item belongs
to a model explicit.

Change-Id: Ib58889fd10e9fce1590a42ade6d847aee747ac56
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/libs/utils/treemodel.cpp

@@ -682,6 +682,7 @@ void TreeItem::appendChild(TreeItem *item)
 
 void TreeItem::insertChild(int pos, TreeItem *item)
 {
+    QTC_CHECK(!item->model());
     QTC_CHECK(!item->parent());
     QTC_ASSERT(0 <= pos && pos <= m_children.size(), return); // '<= size' is intentional.