src: avoid hanging on Buffer#fill 0-length input

Previously, zero-length Buffers and TypedArrays passed as fillers hanged
Buffer#fill and Buffer.from.

This changes those cases when it hanged to a zero-fill instead, which
should be backwards compatible.

This fixes CVE-2018-7167.

PR-URL: https://github.com/nodejs-private/node-private/pull/120
Fixes: https://github.com/nodejs-private/security/issues/193
Refs: 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: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>