buffer: zero fill Buffer(num) by default

PR-URL: https://github.com/nodejs/node/pull/12141
Ref: https://github.com/nodejs/CTC/issues/89


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>