buffer: throw if both length and enc are passed

PR-URL: https://github.com/nodejs/node/pull/4514


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>