nil pointer fix

Author: ucwong

ctxc/sync.go

@@ -172,6 +172,10 @@ func (pm *ProtocolManager) synchronise(peer *peer) {
 	td := pm.blockchain.GetTd(currentBlock.Hash(), currentBlock.NumberU64())
 
 	pHead, pTd := peer.Head()
+	if pTd == nil {
+		return
+	}
+
 	if pTd.Cmp(td) <= 0 {
 		return
 	}