buffer: ensure zero-fill for Buffer.alloc(size,'')

This is applicable to v4.x only.

Native Fill method is called from Buffer.alloc and from Buffer#fill,
the second one is not affected by this, as Buffer#fill only calls the
native method on either numbers as the second argument or
non-zero-length strings.

Fixes: https://github.com/nodejs-private/security/issues/192
PR-URL: https://github.com/nodejs-private/node-private/pull/118


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>