src: assert that uv_async_init() succeeds

This commit fixes two coverity warnings for unchecked
return values.

Refs: https://github.com/nodejs/node/pull/13050
PR-URL: https://github.com/nodejs/node/pull/13116


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>