routing: improve lasthoppaylaod size calculation

Fixes a bug and makes the function more robust. Before
we would always return the encrypted data size of last hop
of the last path. Now we return the greatest last hop payload
not always the one of the last path.

routing/blinding.go

@@ -235,21 +235,33 @@ func (s *BlindedPaymentPathSet) FinalCLTVDelta() uint16 {
 // LargestLastHopPayloadPath returns the BlindedPayment in the set that has the
 // largest last-hop payload. This is to be used for onion size estimation in
 // path finding.
-func (s *BlindedPaymentPathSet) LargestLastHopPayloadPath() *BlindedPayment {
+func (s *BlindedPaymentPathSet) LargestLastHopPayloadPath() (*BlindedPayment,
+	error) {
+
 	var (
 		largestPath *BlindedPayment
 		currentMax  int
 	)
+
+	if len(s.paths) == 0 {
+		return nil, fmt.Errorf("no blinded paths in the set")
+	}
+
+	// We set the largest path to make sure we always return a path even
+	// if the cipher text is empty.
+	largestPath = s.paths[0]
+
 	for _, path := range s.paths {
 		numHops := len(path.BlindedPath.BlindedHops)
 		lastHop := path.BlindedPath.BlindedHops[numHops-1]
 
 		if len(lastHop.CipherText) > currentMax {
 			largestPath = path
+			currentMax = len(lastHop.CipherText)
 		}
 	}
 
-	return largestPath
+	return largestPath, nil
 }
 
 // ToRouteHints converts the blinded path payment set into a RouteHints map so