Avoid warning about missing return value in wxCheckBox code

Handle unknown checkbox values as well, if only to silence the compiler.

src/qt/checkbox.cpp

@@ -119,6 +119,9 @@ wxCheckBoxState wxCheckBox::DoGet3StateValue() const
     case Qt::PartiallyChecked:
         return wxCHK_UNDETERMINED;
     }
+
+    wxFAIL_MSG( "unknown QCheckBox state" );
+    return wxCHK_UNDETERMINED;
 }
 
 QCheckBox *wxCheckBox::GetHandle() const