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/pathfind.go

@@ -700,7 +700,10 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 
 	// The payload size of the final hop differ from intermediate hops
 	// and depends on whether the destination is blinded or not.
-	lastHopPayloadSize := lastHopPayloadSize(r, finalHtlcExpiry, amt)
+	lastHopPayloadSize, err := lastHopPayloadSize(r, finalHtlcExpiry, amt)
+	if err != nil {
+		return nil, 0, err
+	}
 
 	// We can't always assume that the end destination is publicly
 	// advertised to the network so we'll manually include the target node.
@@ -1433,11 +1436,15 @@ func getProbabilityBasedDist(weight int64, probability float64,
 // It depends on the tlv types which are present and also whether the hop is
 // part of a blinded route or not.
 func lastHopPayloadSize(r *RestrictParams, finalHtlcExpiry int32,
-	amount lnwire.MilliSatoshi) uint64 {
+	amount lnwire.MilliSatoshi) (uint64, error) {
 
 	if r.BlindedPaymentPathSet != nil {
-		paymentPath := r.BlindedPaymentPathSet.
+		paymentPath, err := r.BlindedPaymentPathSet.
 			LargestLastHopPayloadPath()
+		if err != nil {
+			return 0, err
+		}
+
 		blindedPath := paymentPath.BlindedPath.BlindedHops
 		blindedPoint := paymentPath.BlindedPath.BlindingPoint
 
@@ -1452,7 +1459,7 @@ func lastHopPayloadSize(r *RestrictParams, finalHtlcExpiry int32,
 		}
 
 		// The final hop does not have a short chanID set.
-		return finalHop.PayloadSize(0)
+		return finalHop.PayloadSize(0), nil
 	}
 
 	var mpp *record.MPP
@@ -1478,7 +1485,7 @@ func lastHopPayloadSize(r *RestrictParams, finalHtlcExpiry int32,
 	}
 
 	// The final hop does not have a short chanID set.
-	return finalHop.PayloadSize(0)
+	return finalHop.PayloadSize(0), nil
 }
 
 // overflowSafeAdd adds two MilliSatoshi values and returns the result. If an