net/mlx5e: TC, Add null pointer check for hardware miss support

The cited commits add hardware miss support to tc action. But if
the rules can't be offloaded, the pointers are null and system
will panic when accessing them.

Fix it by checking null pointer.

Fixes: 08fe94e ("net/mlx5e: TC, Remove special handling of CT action")
Fixes: 6702782 ("net/mlx5e: TC, Set CT miss to the specific ct action instance")
Signed-off-by: Chris Mi <cmi@nvidia.com>
Reviewed-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

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

@@ -2021,6 +2021,8 @@ void
 mlx5_tc_ct_delete_flow(struct mlx5_tc_ct_priv *priv,
 		       struct mlx5_flow_attr *attr)
 {
+	if (!attr->ct_attr.ft) /* no ct action, return */
+		return;
 	if (!attr->ct_attr.nf_ft) /* means only ct clear action, and not ct_clear,ct() */
 		return;