[php-src] PR #25315: Fix GH-25307: NULL ps_files deref after SessionHandler::open() on user module

Pull Request: https://github.com/php/php-src/pull/25315 Author: lazerg After session.save_handler is switched back from "user" to "files" with ini_set(), PS(default_mod) points at the user module. SessionHandler::open() then calls the user open handler, which sets PS(mod_user_implemented) even though files is the active module. Once the files handler has closed the session, RSHUTDOWN still calls its close handler because of that flag, and ps_files_close() gets NULL data. SessionHandler now throws "Cannot call default session handler" when the default module is the user module, since it only wraps a non-user handler. Fixes #25307