i40e: Fix potential invalid access when MAC list is empty

list_first_entry() never returns NULL - if the list is empty, it still
returns a pointer to an invalid object, leading to potential invalid
memory access when dereferenced.

Fix this by using list_first_entry_or_null instead of list_first_entry.

Fixes: e3219ce ("i40e: Add support for client interface for IWARP driver")
Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Author: nizhen-t

drivers/net/ethernet/intel/i40e/i40e_client.c

@@ -359,8 +359,8 @@ static void i40e_client_add_instance(struct i40e_pf *pf)
 	if (i40e_client_get_params(vsi, &cdev->lan_info.params))
 		goto free_cdev;
 
-	mac = list_first_entry(&cdev->lan_info.netdev->dev_addrs.list,
-			       struct netdev_hw_addr, list);
+	mac = list_first_entry_or_null(&cdev->lan_info.netdev->dev_addrs.list,
+				       struct netdev_hw_addr, list);
 	if (mac)
 		ether_addr_copy(cdev->lan_info.lanmac, mac->addr);
 	else