[php-src] PR #24501: ext/standard: Fix wrong base in url rewrite-var separator check

Pull Request: https://github.com/php/php-src/pull/24501 Author: iliaal Corrects a cross-object pointer subtraction in php_url_scanner_reset_var_impl(): start points into url_app.s, but its offset was measured against PG(arg_separator).output, so the preceding-separator guard relies on undefined behaviour and all but always passes. Base the offset on the buffer start actually points into and assert it lies within. No in-tree caller reaches the guard, so there is no observable change; it targets master as a correctness fix.