sessprep: fix detection of unbracketed IPv6 literals.

The centralisation I did in commit e3796cb introduced a foolish
sense error, in which I was supposed to be treating an unbracketed
colon in CONF_hostname as separating host name from a port-number-
shaped suffix if it _was_ the only one, and instead, accidentally did
so if it _wasn't_.

Author: sgtatham

sessprep.c

@@ -63,7 +63,7 @@ void prepare_session(Conf *conf)
      * _more_ than one (un-IPv6-bracketed) colon.
      */
     p = host_strchr(host, ':');
-    if (p && p != host_strrchr(host, ':')) {
+    if (p && p == host_strrchr(host, ':')) {
         *p = '\0';
     }