http2: fix premature destroy

Check stream._writableState.finished instead of stream.writable
as the latter can lead to premature calls to destroy and dropped
writes on busy processes.

PR-URL: https://github.com/nodejs/node/pull/21051
Fixes: https://github.com/nodejs/node/issues/20750


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>