minor cleanup (don't use magic numbers)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@33174 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/checkbox.cpp

@@ -220,7 +220,7 @@ void wxCheckBox::SetValue(bool val)
 
 bool wxCheckBox::GetValue() const
 {
-    return (Get3StateValue() != 0);
+    return (Get3StateValue() != wxCHK_UNCHECKED);
 }
 
 void wxCheckBox::Command(wxCommandEvent& event)