buffer: avoid undefined behaviour

Avoid 'delete this' as it can be hazardous and/or dependent on
implementations.

PR-URL: https://github.com/nodejs/node/pull/5494


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: targos - Michaël Zasso <mic.besace@gmail.com>