[php-src] PR #24538: Fix GH-24532: Reset unserialize var entries before running destructors

Pull Request: https://github.com/php/php-src/pull/24538 Author: lazerg var_destroy() frees the extra var_entries blocks but leaves last and entries.next pointing at them, and only then releases the dtor values. If one of those destructors calls unserialize(), the nested call shares the context that is being torn down and pushes into freed memory. The context is now reset to its embedded block before the destructors run, and any block a nested call allocates there is freed at the end. This only hardens the shared context. GH-21824 / #21828 covers not sharing it during cleanup at all. Fixes GH-24532