htlcswitch: handle nil circuit properly when settling

We have two sources which can call `handlePacketSettle`, either through
the link's `<-s.htlcPlex`, or the `<-s.resolutionMsgs`, which means the
`closeCircuit` could be call twice. Previously we already caught this
case inside `closeCircuit`, in that we would return a nil circuit upon
seeing `ErrUnknownCircuit`, indicating the circuit was removed. However,
we still need to account the case when the circuit is the process of
being closed, which is now fixed as we will ignore when seeing
`ErrCircuitClosing`.

htlcswitch/packet.go

@@ -1,6 +1,8 @@
 package htlcswitch
 
 import (
+	"fmt"
+
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
@@ -136,3 +138,12 @@ func (p *htlcPacket) keystone() Keystone {
 		OutKey: p.outKey(),
 	}
 }
+
+// String returns a human-readable description of the packet.
+func (p *htlcPacket) String() string {
+	return fmt.Sprintf("keystone=%v, sourceRef=%v, destRef=%v, "+
+		"incomingAmount=%v, amount=%v, localFailure=%v, hasSource=%v "+
+		"isResolution=%v", p.keystone(), p.sourceRef, p.destRef,
+		p.incomingAmount, p.amount, p.localFailure, p.hasSource,
+		p.isResolution)
+}