http2: don't send trailers on a closed connection

There is a race condition between onStreamCloseResponse(), which
removes the wantTrailers listener, and Http2Stream.close(), which
will invalidate the connection. IE, sendTrailers can be called on
a closed connection which would crash with a:
Error [ERR_HTTP2_INVALID_STREAM]: The stream has been destroyed

PR-URL: https://github.com/nodejs/node/pull/23146


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>