http2: avoid unnecessary buffer resize

Refs: https://github.com/nodejs/node/pull/34315
Refs: https://github.com/nodejs/node/pull/30351

PR-URL: https://github.com/nodejs/node/pull/34480


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>