nts: don't assume field position in NNA_DecryptAuthEF()

Modify NNA_DecryptAuthEF() to not assume that the authenticator is the
last extension field in the packet as some extension fields specified in
future may need to be placed after the authenticator. The caller of the
function is supposed to verify the position.

nts_ntp_server.c

@@ -135,6 +135,11 @@ NNS_CheckRequestAuth(NTP_Packet *packet, NTP_PacketInfo *info, uint32_t *kod)
         cookie_length = ef_body_length;
         break;
       case NTP_EF_NTS_AUTH_AND_EEF:
+        if (parsed + ef_length != info->length) {
+          DEBUG_LOG("Auth not last EF");
+          return 0;
+        }
+
         auth_start = parsed;
         has_auth = 1;
         break;