etcdserver: check IsMemberExist before IsLearner

If member does not exist in cluster, IsLearner will panic.

etcdserver/server.go

@@ -2518,3 +2518,8 @@ func (s *EtcdServer) Logger() *zap.Logger {
 func (s *EtcdServer) IsLearner() bool {
 	return s.cluster.IsLocalMemberLearner()
 }
+
+// IsMemberExist returns if the member with the given id exists in cluster.
+func (s *EtcdServer) IsMemberExist(id types.ID) bool {
+	return s.cluster.IsMemberExist(id)
+}