don't crash if IsVisible() is called for the hidden virtual root

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@17642 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/treectrl.cpp

@@ -1240,6 +1240,12 @@ void wxTreeCtrl::SetItemFont(const wxTreeItemId& item, const wxFont& font)
 
 bool wxTreeCtrl::IsVisible(const wxTreeItemId& item) const
 {
+    if ( item == wxTreeItemId(TVI_ROOT) )
+    {
+        // virtual (hidden) root is never visible
+        return FALSE;
+    }
+
     // Bug in Gnu-Win32 headers, so don't use the macro TreeView_GetItemRect
     RECT rect;