http2: fix memory leak when headers are not emitted

When headers are not emitted to JS, e.g. because of an error
before that could happen, we currently still have the vector of
previously received headers lying around, each one holding
a reference count of 1.

To fix the resulting memory leak, release them in the `Http2Stream`
destructor.

Also, clear the vector of headers once they have been emitted –
there’s not need to keep it around, wasting memory.

PR-URL: https://github.com/nodejs/node/pull/21373


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>