Check the validity of ECChain first in chain exchange (#837)

Add `ECChain` validation to chain exchange pubsub validation function to
reject invalid chains as early as possible.

Change the ordering of validation rules such that cheaper checks are
done first before checking current progress.

chainexchange/pubsub.go

@@ -209,15 +209,21 @@ func (p *PubSubChainExchange) validatePubSubMessage(_ context.Context, _ peer.ID
 		log.Debugw("failed to decode message", "from", msg.GetFrom(), "err", err)
 		return pubsub.ValidationReject
 	}
+	if cmsg.Chain.IsZero() {
+		// No peer should broadcast a zero-length chain.
+		return pubsub.ValidationReject
+	}
+	if err := cmsg.Chain.Validate(); err != nil {
+		// Invalid chain.
+		log.Debugw("Invalid chain", "from", msg.GetFrom(), "err", err)
+		return pubsub.ValidationReject
+	}
 	switch current := p.progress(); {
 	case
 		cmsg.Instance < current.ID,
 		cmsg.Instance > current.ID+p.maxInstanceLookahead:
 		// Too far ahead or too far behind.
 		return pubsub.ValidationIgnore
-	case cmsg.Chain.IsZero():
-		// No peer should broadcast a zero-length chain.
-		return pubsub.ValidationReject
 	}
 	// TODO: wire in the current base chain from an on-going instance to further
 	//       tighten up validation.