routing: prune channels only if both edges are present

We do this to ensure we don't prune too aggressively, as it's possible
that we've only received the channel announcement for a channel, but not
its accompanying channel updates.

routing/router.go

@@ -638,7 +638,7 @@ func (r *ChannelRouter) pruneZombieChans() error {
 		// If *both* edges haven't been updated for a period of
 		// chanExpiry, then we'll mark the channel itself as eligible
 		// for graph pruning.
-		e1Zombie, e2Zombie := true, true
+		var e1Zombie, e2Zombie bool
 		if e1 != nil {
 			e1Zombie = time.Since(e1.LastUpdate) >= chanExpiry
 			if e1Zombie {