multi: update PaymentAddr to use fn.Option

Since it is allowed to not be set and so can lead to nil deref panics if
it is a pointer.

routing/pathfind.go

@@ -111,7 +111,7 @@ type finalHopParams struct {
 	cltvDelta uint16
 
 	records     record.CustomSet
-	paymentAddr *[32]byte
+	paymentAddr fn.Option[[32]byte]
 
 	// metadata is additional data that is sent along with the payment to
 	// the payee.
@@ -226,7 +226,7 @@ func newRoute(sourceVertex route.Vertex,
 			// If we're attaching a payment addr but the receiver
 			// doesn't support both TLV and payment addrs, fail.
 			payAddr := supports(lnwire.PaymentAddrOptional)
-			if !payAddr && finalHop.paymentAddr != nil {
+			if !payAddr && finalHop.paymentAddr.IsSome() {
 				return nil, errors.New("cannot attach " +
 					"payment addr")
 			}
@@ -234,12 +234,9 @@ func newRoute(sourceVertex route.Vertex,
 			// Otherwise attach the mpp record if it exists.
 			// TODO(halseth): move this to payment life cycle,
 			// where AMP options are set.
-			if finalHop.paymentAddr != nil {
-				mpp = record.NewMPP(
-					finalHop.totalAmt,
-					*finalHop.paymentAddr,
-				)
-			}
+			finalHop.paymentAddr.WhenSome(func(addr [32]byte) {
+				mpp = record.NewMPP(finalHop.totalAmt, addr)
+			})
 
 			metadata = finalHop.metadata
 
@@ -452,7 +449,7 @@ type RestrictParams struct {
 	// PaymentAddr is a random 32-byte value generated by the receiver to
 	// mitigate probing vectors and payment sniping attacks on overpaid
 	// invoices.
-	PaymentAddr *[32]byte
+	PaymentAddr fn.Option[[32]byte]
 
 	// Amp signals to the pathfinder that this payment is an AMP payment
 	// and therefore it needs to account for additional AMP data in the
@@ -608,7 +605,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 	// checking that it supports the features we need. If the caller has a
 	// payment address to attach, check that our destination feature vector
 	// supports them.
-	if r.PaymentAddr != nil &&
+	if r.PaymentAddr.IsSome() &&
 		!features.HasFeature(lnwire.PaymentAddrOptional) {
 
 		return nil, 0, errNoPaymentAddr
@@ -1435,9 +1432,9 @@ func lastHopPayloadSize(r *RestrictParams, finalHtlcExpiry int32,
 	}
 
 	var mpp *record.MPP
-	if r.PaymentAddr != nil {
-		mpp = record.NewMPP(amount, *r.PaymentAddr)
-	}
+	r.PaymentAddr.WhenSome(func(addr [32]byte) {
+		mpp = record.NewMPP(amount, addr)
+	})
 
 	var amp *record.AMP
 	if r.Amp != nil {