stream: avoid writeAfterEnd() while ending

Calling `writable.end()` will probably synchronously call
`writable.write()`, in such a situation the `state.ended`
is false and `writable.write()` doesn't trigger `writeAfterEnd()`.

PR-URL: https://github.com/nodejs/node/pull/18170


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