telnetd: fix two-byte input crash

Move initialization of the slc table earlier so it doesn't get
accessed before that happens.

For details on the issue, see:
https://pierrekim.github.io/blog/2022-08-24-2-byte-dos-freebsd-netbsd-telnetd-netkit-telnetd-inetutils-telnetd-kerberos-telnetd.html

Reviewed by: cy
Obtained from: NetBSD via cy
Differential Revision: https://reviews.freebsd.org/D36680