tapchannel: aux invoice manager rejects asset payment on btc invoice

Previously we had strict-forwarding protection placed in the
AuxInvoiceManager, but that would only protect asset invoices against
pure-btc HTLCs. This commit adds the inverse complementary check which
prevents settling the invoice if btc ras requested but assets were
offered.

Author: GeorgeTsagk

tapchannel/aux_invoice_manager.go

@@ -164,6 +164,13 @@ func (s *AuxInvoiceManager) handleInvoiceAccept(_ context.Context,
 			resp.CancelSet = true
 		}
 
+		return resp, nil
+	} else if !isAssetInvoice(req.Invoice, s) && !req.Invoice.IsKeysend {
+		// If we do have custom records, but the invoice does not
+		// correspond to an asset invoice, we do not settle the invoice.
+		// Since we requested btc we should be receiving btc.
+		resp.CancelSet = true
+
 		return resp, nil
 	}