invoices: always check htlc amt with invoice amount

Previously a check was made for accepted and settled invoices against
the paid amount. This opens up a probe vector where an attacker can pay
to an invoice with an amt that is higher than the invoice amount and
find out if the invoice is already paid or not.

invoices/invoiceregistry_test.go

@@ -165,9 +165,9 @@ func TestSettleInvoice(t *testing.T) {
 		t.Fatal("expected settle event")
 	}
 
-	// Try to settle again with a higher amount. This should result in a
-	// cancel event because after a restart the amount should still be the
-	// same. New HTLCs with a different amount should be rejected.
+	// Try to settle again with a higher amount. This payment should also be
+	// accepted, to prevent any change in behaviour for a paid invoice that
+	// may open up a probe vector.
 	event, err = registry.NotifyExitHopHtlc(
 		hash, amtPaid+600, testHtlcExpiry, testCurrentHeight,
 		hodlChan, nil,
@@ -175,12 +175,12 @@ func TestSettleInvoice(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unexpected NotifyExitHopHtlc error: %v", err)
 	}
-	if event.Preimage != nil {
-		t.Fatal("expected cancel event")
+	if event.Preimage == nil {
+		t.Fatal("expected settle event")
 	}
 
-	// Try to settle again with a lower amount. This should show the same
-	// behaviour as settling with a higher amount.
+	// Try to settle again with a lower amount. This should fail just as it
+	// would have failed if it were the first payment.
 	event, err = registry.NotifyExitHopHtlc(
 		hash, amtPaid-600, testHtlcExpiry, testCurrentHeight,
 		hodlChan, nil,