The result of nlmsg_find_attr() 'br_spec' is dereferenced in
nla_for_each_nested(), but it can take NULL value in nla_find() function,
which will result in an error.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 51616018 ("i40e: Add support for getlink, setlink ndo ops")
Signed-off-by: NNatalia Petrova <n.petrova@fintech.ru>
Reviewed-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>
Link: https://lore.kernel.org/r/20230209172833.3596034-1-anthony.l.nguyen@intel.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>