[487178] Obey mount_point when publishing wills.

Wills published by clients connected to a listener with mount_point defined
now correctly obey the mount point. This was a potential security risk
because it allowed clients to publish messages outside of their restricted
mount point. This is only affects brokers where the mount_point option is in
use.

Thanks to Lance Riley.

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=487178