lnwire: add update_fail_malformed_htlc message

In this commit BOLT#4 specification message have been added to the
lnwire package. This messsage is needed in order to notify payment
sender that forwarding node unable to parse the onion blob.

lnwire/lnwire_test.go

@@ -529,6 +529,13 @@ func TestLightningWireProtocol(t *testing.T) {
 				return mainScenario(&m)
 			},
 		},
+		{
+
+			msgType: MsgUpdateFailMalformedHTLC,
+			scenario: func(m UpdateFailMalformedHTLC) bool {
+				return mainScenario(&m)
+			},
+		},
 		{
 			msgType: MsgChannelAnnouncement,
 			scenario: func(m ChannelAnnouncement) bool {