fixed handling of the items with indirect data broken yesterday

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@13172 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/treectrl.cpp

@@ -423,6 +423,9 @@ public:
 
         // and set ourselves as the new one
         tree->SetIndirectItemData(item, this);
+
+        // we must have the invalid value for the item
+        m_pItem = 0l;
     }
 
     // dtor deletes the associated data as well