net/mlx5e: Fix null deref accessing lag dev

It could be the lag dev is null so stop processing the event.
In bond_enslave() the active/backup slave being set before setting the
upper dev so first event is without an upper dev.
After setting the upper dev with bond_master_upper_dev_link() there is
a second event and in that event we have an upper dev.

Fixes: 7e51891a ("net/mlx5e: Use netdev events to set/del egress acl forward-to-vport rule")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en/rep/bond.c

@@ -223,6 +223,8 @@ static void mlx5e_rep_changelowerstate_event(struct net_device *netdev, void *pt
 	rpriv = priv->ppriv;
 	fwd_vport_num = rpriv->rep->vport;
 	lag_dev = netdev_master_upper_dev_get(netdev);
+	if (!lag_dev)
+		return;
 
 	netdev_dbg(netdev, "lag_dev(%s)'s slave vport(%d) is txable(%d)\n",
 		   lag_dev->name, fwd_vport_num, net_lag_port_dev_txable(netdev));