buffer: coerce offset to integer

The offset was formerly coerced to a integer and this reimplements
that.

PR-URL: https://github.com/nodejs/node/pull/18215
Fixes: https://github.com/nodejs/node/issues/18208


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>