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.

zpay32/encode.go

@@ -9,6 +9,7 @@ import (
 	"github.com/btcsuite/btcd/btcutil/bech32"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -301,14 +302,14 @@ func writeTaggedFields(bufferBase32 *bytes.Buffer, invoice *Invoice) error {
 			return err
 		}
 	}
-	if invoice.PaymentAddr != nil {
-		err := writeBytes32(
-			bufferBase32, fieldTypeS, *invoice.PaymentAddr,
-		)
-		if err != nil {
-			return err
-		}
+
+	err := fn.MapOptionZ(invoice.PaymentAddr, func(addr [32]byte) error {
+		return writeBytes32(bufferBase32, fieldTypeS, addr)
+	})
+	if err != nil {
+		return err
 	}
+
 	if invoice.Features.SerializeSize32() > 0 {
 		var b bytes.Buffer
 		err := invoice.Features.RawFeatureVector.EncodeBase32(&b)