http2: fix memory leak for uncommon headers

Fix a memory leak that occurs with header names that are
short and not present in the static table of default headers.

PR-URL: https://github.com/nodejs/node/pull/21336


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>