buffer: stricter argument checking in toString

This prevents the confusing behavior of `buf.toString(0, 5)` by
disallowing passing `0` as the encoding.

PR-URL: https://github.com/nodejs/node/pull/11120


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>