net/mlx5: check attr pointer validity before dereferencing it

Fix attr pointer validity checks after it was already
dereferenced.

Fixes: cb0d54cb ("net/mlx5e: Fix wrong source vport matching on tunnel rule")
Signed-off-by: Ariel Levkovich <lariel@nvidia.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>