net/mlx5: fix null dereference in modify header

net/mlx5: fix null dereference in modify header

GCC analyzer identified a code path where acts->mhdr could be
NULL when dereferenced.
When modify header validation fails in mlx5_tbl_translate_modify_header(),
 __flow_hw_action_template_destroy() sets acts->mhdr to NULL.
Add defensive NULL check in mlx5_tbl_ensure_shared_modify_header()
to prevent the dereference.

Bugzilla ID: 1521
Fixes: 12f2ed3f03c8 ("net/mlx5: set modify header as shared flow action")
Cc: stable@dpdk.org
Signed-off-by: Shani Peretz <shperetz@nvidia.com>
Acked-by: Dariusz Sosnowski <dsosnowski@nvidia.com>