i40e: check that pointer VSI is not null before dereferencing it

Function i40e_find_vsi_from_id can potentially return null, hence
VSI may be null, so defensively check it is non-null before
dereferencing it to check the seid.

Fixes: e284fc28 ("i40e: Add and delete cloud filter")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Avinash Dayanand <avinash.dayanand@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>