buffer: remove unnecessary argument check

In Buffer.prototype.compare, the first check makes sure that target is
an instance of Buffer. The value cannot be falsy after that so we can
safely get its length.

PR-URL: https://github.com/nodejs/node/pull/8552


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>