Check for non-null window in wxRendererNative::GetCheckBoxSize()

Instead of running normally under some platforms and crashing under MSW
(when using themes) when passing NULL to GetCheckBoxSize(), now
consistently assert and return zero size everywhere.

Closes #18241.

src/generic/renderg.cpp

@@ -714,6 +714,8 @@ wxRendererGeneric::DrawCheckBox(wxWindow *WXUNUSED(win),
 
 wxSize wxRendererGeneric::GetCheckBoxSize(wxWindow *win)
 {
+    wxCHECK_MSG( win, wxSize(0, 0), "Must have a valid window" );
+
     return win->FromDIP(wxSize(16, 16));
 }