right clicking outside any column sets the column in the list event to -1, not the last column

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@14554 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/listctrl/listtest.cpp

@@ -631,7 +631,10 @@ void MyListCtrl::OnColClick(wxListEvent& event)
 void MyListCtrl::OnColRightClick(wxListEvent& event)
 {
     int col = event.GetColumn();
-    SetColumnImage(col, -1);
+    if ( col != -1 )
+    {
+        SetColumnImage(col, -1);
+    }
 
     wxLogMessage( wxT("OnColumnRightClick at %d."), event.GetColumn() );
 }