WNM: Verify WNM Sleep Mode element length

This element is required to have at least four octets of actual payload.
This was not previously verified before use and the extra buffer data
after the IE might have been used instead if a received WNM-Sleep Mode
Response frame was invalid.

Signed-off-by: Jouni Malinen <j@w1.fi>

wpa_supplicant/wnm_sta.c

@@ -268,7 +268,7 @@ static void ieee802_11_rx_wnmsleep_resp(struct wpa_supplicant *wpa_s,
 			break;
 		}
 		wpa_hexdump(MSG_DEBUG, "WNM: Element", pos, 2 + ie_len);
-		if (*pos == WLAN_EID_WNMSLEEP)
+		if (*pos == WLAN_EID_WNMSLEEP && ie_len >= 4)
 			wnmsleep_ie = (struct wnm_sleep_element *) pos;
 		else if (*pos == WLAN_EID_TFS_RESP) {
 			if (!tfsresp_ie_start)