[php-src] Issue #7942: Readonly properties mutable when assigning reference to them

Issue: https://github.com/php/php-src/issues/7942 Comment Author: nikic In zend_std_get_property_ptr_ptr(), we currently allow acquiring a property pointer if the current value is undef and the scope has initialization access. This then allows zend_assign_to_property_reference() to assign a reference to the property.