route: return 0 fee from TotalFees for empty route.

Otherwise we would crash calling the method on empty routes.

routing/route/route.go

@@ -103,6 +103,10 @@ func (r *Route) HopFee(hopIndex int) lnwire.MilliSatoshi {
 // the case of a one-hop payment, this value will be zero as we don't need to
 // pay a fee to ourself.
 func (r *Route) TotalFees() lnwire.MilliSatoshi {
+	if len(r.Hops) == 0 {
+		return 0
+	}
+
 	return r.TotalAmount - r.Hops[len(r.Hops)-1].AmtToForward
 }