napi: fix memory corruption vulnerability

Fixes: https://hackerone.com/reports/784186
CVE-ID: CVE-2020-8174
PR-URL: https://github.com/nodejs-private/node-private/pull/203


Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>