Fix potential null-pointer dereference in ssh_reconfig.

ssh->base_layer is NULL when the connection is still in its early
stages, before greetings are exchanged. If the user invokes the Change
Settings dialog in this situation, ssh_reconfig would call
ssh_ppl_reconfigure() on ssh->base_layer without checking if it was
NULL first.

(cherry picked from commit d67c13eeb8)

ssh/ssh.c

@@ -1038,6 +1038,7 @@ static void ssh_reconfig(Backend *be, Conf *conf)
     if (ssh->pinger)
         pinger_reconfig(ssh->pinger, ssh->conf, conf);
 
+    if (ssh->base_layer)
         ssh_ppl_reconfigure(ssh->base_layer, conf);
 
     conf_free(ssh->conf);