stream: fix `Writable` subclass instanceof checks

2a4b068a introduced a regression in where checking
`instanceof` would fail for `Writable` subclasses inside the
subclass constructor, i.e. before `Writable()` was called.

Also, calling `null instanceof Writable` or
`undefined instanceof Writable` would fail due to accessing the
`_writableState` property of the target object.

This fixes these problems.

PR-URL: https://github.com/nodejs/node/pull/9088
Ref: https://github.com/nodejs/node/pull/8834#issuecomment-253640692


Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>