EAP-TNC: Accept fragment ack frame with Flags field

TNC IF-T specification is unclear on the exact contents of the fragment
acknowledgement frame. An interoperability issue with the tncs@fhh
implementation was reported by Arne Welzel
<arne.welzel@stud.fh-hannover.de> due to the different interpretations
of the specification. Relax EAP-TNC server/peer validation rules to
accept fragmentation acknowledgement frames to include the Flags field
to avoid this issue.

src/eap_peer/eap_tnc.c

@@ -262,7 +262,7 @@ static struct wpabuf * eap_tnc_process(struct eap_sm *sm, void *priv,
 		   "Message Length %u", flags, message_length);
 
 	if (data->state == WAIT_FRAG_ACK) {
-		if (len != 0) {
+		if (len > 1) {
 			wpa_printf(MSG_DEBUG, "EAP-TNC: Unexpected payload in "
 				   "WAIT_FRAG_ACK state");
 			ret->ignore = TRUE;

src/eap_server/eap_tnc.c

@@ -453,7 +453,7 @@ static void eap_tnc_process(struct eap_sm *sm, void *priv,
 		   "Message Length %u", flags, message_length);
 
 	if (data->state == WAIT_FRAG_ACK) {
-		if (len != 0) {
+		if (len > 1) {
 			wpa_printf(MSG_DEBUG, "EAP-TNC: Unexpected payload "
 				   "in WAIT_FRAG_ACK state");
 			data->state = FAIL;