Avoids asserts in the listctrl sample on pressing Ctrl-I

Calling InsertItemInReportView() with invalid index results in a series
of asserts, so avoid doing this.

Note that there seems to be an unrelated bug in wxGTK which results in
this code being executed at all, as normally the definition of a menu
item using Ctrl-I as an accelerator should have prevented this from
happening.

samples/listctrl/listtest.cpp

@@ -1435,7 +1435,10 @@ void MyListCtrl::OnListKeyDown(wxListEvent& event)
                 }
                 else // !virtual
                 {
-                    InsertItemInReportView(event.GetIndex());
+                    int idx = event.GetIndex();
+                    if ( idx == -1 )
+                        idx = 0;
+                    InsertItemInReportView(idx);
                 }
                 break;
             }