buffer: neuter external `nullptr` buffers

Neuter external `nullptr` buffers, otherwise their contents will be
materialized on access, and the buffer instance will be internalized.

This leads to a crash like this:

    v8::ArrayBuffer::Neuter Only externalized ArrayBuffers can be
    neutered

Fix: #3619
PR-URL: https://github.com/nodejs/node/pull/3624


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>