iwlwifi: check scan request ie_len

In mac80211 we always check both scan_req->ie and scan_req->ie_len
against zero before usage, in iwlwifi we should do the same.

Remove not needed "left -= ie_len" while at it.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>