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

contrib/telnet/telnetd/telnetd.c

@@ -658,6 +658,11 @@ doit(struct sockaddr *who)
 	int err_; /* XXX */
 	int ptynum;
 
+	/*
+	 * Initialize the slc mapping table.
+	 */
+	get_slc_defaults();
+
 	/*
 	 * Find an available pty to use.
 	 */
@@ -741,11 +746,6 @@ telnet(int f, int p, char *host)
 	struct stat statbuf;
 	int nfd;
 
-	/*
-	 * Initialize the slc mapping table.
-	 */
-	get_slc_defaults();
-
 	/*
 	 * Do some tests where it is desireable to wait for a response.
 	 * Rather than doing them slowly, one at a time, do them all