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>
