src: only set JSStreamWrap write req after `write()`

Otherwise `this[kCurrentWriteRequest]` is set to a value even
if one of the `write` calls throws.

This is needed in order not to break tests in a later commit.

PR-URL: https://github.com/nodejs/node/pull/18676


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>