Fix the most obvious of the bugs introduced by recent syncache changes

- *ip is not initialized in the case of inet6 connection, but ip->ip_len is
  being changed anyway

Now the question is, why does it think an ipv4 connection is an ipv6 connection?
xemacs still doesn't work over X11 forwarding, but the kernel no longer panics.

sys/netinet/tcp_syncache.c

@@ -1225,8 +1225,11 @@ syncache_respond(struct syncache *sc, struct mbuf *m)
 #ifdef INET6
 		if (sc->sc_inc.inc_isipv6)
 			ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
+		else
 #endif
+		{
 			ip->ip_len += optlen;
+		}
 	} else
 		optlen = 0;