message: Drop fragments with total fragment count lower than before

The RFC only allows that the number of fragments increases (if the
sender reduces the MTU).

Not enforcing this before could cause early reassembly as the trigger was
that the number of received fragments matches the total count of the
current packet (which was a bit weird anyway).  Only an active MITM could
trigger this as individual fragments are encrypted and authenticated.

src/libcharon/encoding/message.c

@@ -3054,6 +3054,10 @@ METHOD(message_t, add_fragment_v2, status_t,
 		this->frag->last = total;
 		this->fragments = array_create(0, total);
 	}
+	else if (total < this->frag->last)
+	{	/* drop these silently as mandated by RFC 7383, section 2.6 */
+		return INVALID_ARG;
+	}
 	num = encrypted_fragment->get_fragment_number(encrypted_fragment);
 	data = encrypted_fragment->get_content(encrypted_fragment);
 	status = add_fragment(this, num, data);
@@ -3080,11 +3084,11 @@ METHOD(message_t, add_fragment_v2, status_t,
 		enumerator->destroy(enumerator);
 	}
 
-	if (array_count(this->fragments) != total)
+	if (array_count(this->fragments) != this->frag->last)
 	{
 		/* there are some fragments missing */
 		DBG1(DBG_ENC, "received fragment #%hu of %hu, waiting for complete IKE "
-			 "message", num, total);
+			 "message", num, this->frag->last);
 		return NEED_MORE;
 	}