n-api: fix warning about size_t compare with int

PR-URL: https://github.com/nodejs/node/pull/15508


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>