buffer: use a default offset

If none is provided, use zero as a default offset for all read/write
operations on the buffer.

PR-URL: https://github.com/nodejs/node/pull/19749
Refs: https://github.com/nodejs/node/pull/18395


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>