libc/assert: Reference the expression in all case

to avoid the warning "defined but not used"

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: I55b7c092d0f2e5882fc1784987657c10cdf2d90b

wireless/ieee802154/mac802154_netdev.c

@@ -1361,8 +1361,6 @@ int mac802154netdev_register(MACHANDLE mac)
 
   nxsem_init(&priv->md_exclsem, 0, 1);
 
-  DEBUGASSERT(priv->md_txpoll != NULL);
-
   /* Set the network mask. */
 
   macnet_netmask(dev);