buffer: don't abort on prototype getters

Accessing prototype properties directly on a typed array will throw. So
do an extra check in Buffer's own getters to verify it is being called
on an instance.

Fixes: https://github.com/nodejs/node/issues/3297
PR-URL: https://github.com/nodejs/node/pull/3302


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>