net/mlx5e: Make sure fib dev exists in fib event

For unreachable route entry the fib dev does not exists.

Fixes: 8914add2 ("net/mlx5e: Handle FIB events to update tunnel endpoint device")
Reported-by: NDennis Afanasev <dennis.afanasev@stateless.net>
Signed-off-by: NRoi Dayan <roid@nvidia.com>
Reviewed-by: NMaor Dickman <maord@nvidia.com>
Reviewed-by: NVlad Buslov <vladbu@nvidia.com>
Signed-off-by: NSaeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun_encap.c

@@ -1505,7 +1505,7 @@ mlx5e_init_fib_work_ipv4(struct mlx5e_priv *priv,
 
 	fen_info = container_of(info, struct fib_entry_notifier_info, info);
 	fib_dev = fib_info_nh(fen_info->fi, 0)->fib_nh_dev;
-	if (fib_dev->netdev_ops != &mlx5e_netdev_ops ||
+	if (!fib_dev || fib_dev->netdev_ops != &mlx5e_netdev_ops ||
 	    fen_info->dst_len != 32)
 		return NULL;