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/195


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>