http2: fixup http2stream cleanup and other nits

This fixes CVE-2018-7161.

PR-URL: https://github.com/nodejs-private/node-private/pull/115


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>