ggsn_vty.c: fix: use CONFIG_NODE as parent by default

There are some configuration nodes, which are handled by extenral
libraries, such as libosmoctrl. So, when switching back to the
parent node, this should be kept in mind.

Change-Id: I65be7910dc46166caa34a0984a6763e1477dec99

ggsn/ggsn_vty.c

@@ -948,6 +948,10 @@ static int ggsn_vty_go_parent(struct vty *vty)
 			vty->index_sub = &apn->ggsn->cfg.description;
 		}
 		break;
+	default:
+		vty->node = CONFIG_NODE;
+		vty->index = NULL;
+		vty->index_sub = NULL;
 	}
 
 	return vty->node;