Revert "fix: topicscore params can't be set for dynamically subscribe…

…d topic (libp2p#540)" (libp2p#541)

This reverts commit aa5fd79.

score.go

@@ -202,9 +202,13 @@ func (ps *peerScore) SetTopicScoreParams(topic string, p *TopicScoreParams) erro
 	ps.Lock()
 	defer ps.Unlock()
 
-	old := ps.params.Topics[topic]
+	old, exist := ps.params.Topics[topic]
 	ps.params.Topics[topic] = p
 
+	if !exist {
+		return nil
+	}
+
 	// check to see if the counter Caps are being lowered; if that's the case we need to recap them
 	recap := false
 	if p.FirstMessageDeliveriesCap < old.FirstMessageDeliveriesCap {