jail: guard the IP address pointer in kern_jail_get()

[nick_spun.io]

Summary

For a prison with no address of a family, kern_jail_get() evaluated
pr->pr_addrs[af]->pr_ip, a member access on a NULL prison_ip, for
vfs_setopt_part()'s value argument. The length is 0 in that case and
the value then goes unused, so it is harmless, but KUBSAN flags the
undefined access.

Guard the pointer like the length already is.