[php-src] PR #24734: Fix GH-24731: Assertion failure on bailout in header_register_callback()

Pull Request: https://github.com/php/php-src/pull/24734 Author: lazerg header_register_callback() released the previous callback while it was still stored in SG(callback_func). If a destructor run by that release bails out (fatal error, timeout), the slot keeps pointing at the half-freed closure, and the fatal error output goes through sapi_send_headers(), which calls and releases it a second time. The callback is now moved out of the global before it is released, like sapi_send_headers() already does. Fixes #24731