i40e: fix null dereference

If the vsi->tx_rings structure is NULL we don't want to panic.

Change-Id: Ic694f043701738c434e8ebe0caf0673f4410dc10
Signed-off-by: Jesse Brandeburg <[e-mail]>
Tested-by: Kavindya Deegala <[e-mail]>
Signed-off-by: Jeff Kirsher <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/intel/i40e/i40e_main.c

···
 	struct rtnl_link_stats64 *vsi_stats = i40e_get_vsi_stats_struct(vsi);
 	int i;
 
+	if (!vsi->tx_rings)
+		return stats;
+
 	rcu_read_lock();
 	for (i = 0; i < vsi->num_queue_pairs; i++) {
 		struct i40e_ring *tx_ring, *rx_ring;