do not request another call if server is shutdown

src/cpp/server/server.cc

@@ -446,8 +446,12 @@ void Server::RunRpc() {
     ScheduleCallback();
     if (ok) {
       SyncRequest::CallData cd(this, mrd);
+      {
+        grpc::unique_lock<grpc::mutex> lock(mu_);
+        if (!shutdown_) {
           mrd->Request(server_);
-
+        }
+      }
       cd.Run();
     }
   }