Fix cursor flicker after popup menu in rich wxTextCtrl

After bd650ec and cad2b9c, showing a popup menu in wxTextCtrl with
wxTE_RICH2 and then dismissing it without choosing a command would
result in the cursor flickering between arrow and beam cursors until
another context menu was shown.

See https://github.com/vslavik/poedit/issues/483

Fixed by checking if the popup menu is still valid (and so shown) in
the cursor handling code.

src/msw/textctrl.cpp

@@ -2262,9 +2262,14 @@ wxTextCtrl::MSWHandleMessage(WXLRESULT *rc,
             // for plain EDIT controls though), so explicitly work around this
             if ( IsRich() )
             {
+                // wxCurrentPopupMenu stores the popup menu that will receive
+                // WM_COMMAND, but it may be non-NULL even when the underlying
+                // native menu is no longer shown. Use ::IsMenu() to check whether
+                // the menu still exists.
                 extern wxMenu *wxCurrentPopupMenu;
                 if ( wxCurrentPopupMenu &&
-                        wxCurrentPopupMenu->GetInvokingWindow() == this )
+                        wxCurrentPopupMenu->GetInvokingWindow() == this &&
+                        ::IsMenu(GetHmenuOf(wxCurrentPopupMenu)) )
                     ::SetCursor(GetHcursorOf(*wxSTANDARD_CURSOR));
             }
 #endif // wxUSE_MENUS