n-api: fix memory leak in napi_async_destroy()

PR-URL: https://github.com/nodejs/node/pull/17714


Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>