http2: replace unreachable error with assertion

"That particular `emit('error', ...)` is largely defensively coded and
should not ever actually happen." Sounds like an assertion rather than
an error event. The code in question has no test coverage because it is
believed to be unreachable.

Fixes: https://github.com/nodejs/node/issues/20673

PR-URL: https://github.com/nodejs/node/pull/24407


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>