http: refactor agent 'free' handler

Remove nesting in favor of early returns.

PR-URL: https://github.com/nodejs/node/pull/32801


Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>