nts: fix number of extension fields after failed encryption

If the authenticator SIV encryption fails (e.g. due to wrong nonce
length), decrement the number of extension fields to keep the packet
info consistent.

nts_ntp_auth.c

@@ -115,6 +115,7 @@ NNA_GenerateAuthEF(NTP_Packet *packet, NTP_PacketInfo *info, SIV_Instance siv,
                    plaintext, plaintext_length, ciphertext, ciphertext_length)) {
     DEBUG_LOG("SIV encrypt failed");
     info->length = assoc_length;
+    info->ext_fields--;
     return 0;
   }