vty: command.c: Add assert

a program being developed right now crashed at this point, without
providing any meaningful information on where did it crash.

Change-Id: Ia14f43142e7409f72eb9efd5c9131bea5eed6e82

src/vty/command.c

@@ -1092,6 +1092,7 @@ static int config_write_host(struct vty *vty)
 static vector cmd_node_vector(vector v, enum node_type ntype)
 {
 	struct cmd_node *cnode = vector_slot(v, ntype);
+	OSMO_ASSERT(cnode != NULL);
 	return cnode->cmd_vector;
 }