Fix drawing undetermined state of owner-drawn checkbox

src/msw/checkbox.cpp

@@ -247,11 +247,11 @@ int wxCheckBox::MSWGetButtonCheckedFlag() const
             return wxCONTROL_CHECKED;
 
         case wxCHK_UNDETERMINED:
-            return wxCONTROL_PRESSED;
+            return wxCONTROL_UNDETERMINED;
 
         case wxCHK_UNCHECKED:
             // no extra styles needed
-            return 0;
+            return wxCONTROL_NONE;
     }
 
     wxFAIL_MSG( wxT("unexpected Get3StateValue() return value") );