timers: extract enroll() validation into a fn

This should help keep everything consistent.

PR-URL: https://github.com/nodejs/node/pull/17704


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>