Fix stream closed before response during batch stream creation (#3214)

Author: jenrryyou

src/brpc/controller.cpp

@@ -1448,6 +1448,7 @@ void Controller::HandleStreamConnection(Socket *host_socket) {
         auto extra_stream_ids = std::move(*_remote_stream_settings->mutable_extra_stream_ids());
         _remote_stream_settings->clear_extra_stream_ids();
         for (size_t i = 1; i < stream_num; ++i) {
+            if(!ptrs[i]) continue;
             Stream* extra_stream = (Stream *) ptrs[i]->conn();
             _remote_stream_settings->set_stream_id(extra_stream_ids[i - 1]);
             s->SetHostSocket(host_socket);