BUG #15932: Module passwordcheck doesn't reference previous hooks

The following bug has been logged on the website:

Bug reference:      15932
Logged by:          Rafael Castro
Email address:      rafaelthca@gmail.com
PostgreSQL version: 11.4
Operating system:   Any
Description:

Module passwordcheck doesn't reference (thus, doesn't call) previous
hooks:

https://github.com/postgres/postgres/blob/master/contrib/passwordcheck/passwordcheck.c#L133

As a result, if passwordcheck is used hook check_password_hook becomes
useless.