n-api: fix coverity scan report

Coverity was reporting _request.work_req as not being initialized.
Add memset to ensure all of _request is initialized.

PR-URL: https://github.com/nodejs/node/pull/12365


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