trx_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.

Instead of aborting, let's got to the CONFIG_NODE by default.

Fixes: OS#3250
Change-Id: Ia0600a46d19825806e5aed9257b6c57c3907808b

CommonLibs/trx_vty.c

@@ -503,7 +503,9 @@ static int trx_vty_go_parent(struct vty *vty)
 		vty->index_sub = NULL;
 		break;
 	default:
-		OSMO_ASSERT(0);
+		vty->node = CONFIG_NODE;
+		vty->index = NULL;
+		vty->index_sub = NULL;
 	}
 
 	return vty->node;