Find: Stop at the root item when updating the "checked" status

... of search result items.
Otherwise we're breaking assumptions elsewhere.

Fixes: QTCREATORBUG-25662
Change-Id: I9dce79bd58ada02988741741c09e4ce42cc7af03
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/coreplugin/find/searchresulttreemodel.cpp

@@ -262,7 +262,7 @@ bool SearchResultTreeModel::setCheckState(const QModelIndex &idx, Qt::CheckState
 void SearchResultTreeModel::updateCheckStateFromChildren(const QModelIndex &idx,
                                                          SearchResultTreeItem *item)
 {
-    if (!item)
+    if (!item || item == m_rootItem)
         return;
 
     bool hasChecked = false;