http2: avoid race condition in OnHeaderCallback

Fixes: https://github.com/nodejs/node/issues/21416

PR-URL: https://github.com/nodejs/node/pull/22256


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>