Don't periodically check GSS creds in non-GSS mode.

The 2-minutely check to see whether new GSS credentials need to be
forwarded to the server is pointless if we're not even in the mode
where we _have_ forwarded a previous set.

This was made obvious by the overly verbose diagnostic fixed in the
previous commit, so it's a good thing that bug was temporarily there!

ssh.c

@@ -12030,7 +12030,12 @@ static int ssh2_timer_update(Ssh ssh, unsigned long rekey_time)
     }
 
 #ifndef NO_GSSAPI
-    {
+    if (ssh->gss_kex_used) {
+        /*
+         * If we've used GSSAPI key exchange, then we should
+         * periodically check whether we need to do another one to
+         * pass new credentials to the server.
+         */
         unsigned long gssmins;
 
         /* Check cascade conditions more frequently if configured */