fix(nameserver): panic in nameserver size is changed. (#529)

diff --git a/internal/namesrv.go b/internal/namesrv.go
index 6d8ff2b..371b431 100644
--- a/internal/namesrv.go
+++ b/internal/namesrv.go
@@ -120,7 +120,7 @@
 	s.lock.Lock()
 	defer s.lock.Unlock()
 
-	addr := s.srvs[s.index]
+	addr := s.srvs[s.index%len(s.srvs)]
 	index := s.index + 1
 	if index < 0 {
 		index = -index