[php-src] PR #23781: Fix OSS-Fuzz #536440507: Immutable class incorrect assertion

Pull Request: https://github.com/php/php-src/pull/23781 Author: ndossche For immutable classes, the flag for updated constants lives on the mutable part (see zend_update_class_constants). That means the assertion is bogus and can be replaced with a more complex check via a helper function. For the case where we perform the flags check, but not as an assertion but as a proper check, deferring to zend_update_class_constants() is enough because it already checks the flags correctly itself.