http2: do not start reading after write if new write is on wire

Don’t start reading more input data if we’re still busy writing output.
This was overlooked in 8a4a1931.

Fixes: https://github.com/nodejs/node/issues/29353
Fixes: https://github.com/nodejs/node/issues/29393

PR-URL: https://github.com/nodejs/node/pull/29399
Backport-PR-URL: https://github.com/nodejs/node/pull/29618


Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>