nghttpx: Use copy instead of const reference of backend group

src/shrpx_http_downstream_connection.h

@@ -88,7 +88,7 @@ private:
   Worker *worker_;
   // nullptr if TLS is not used.
   SSL_CTX *ssl_ctx_;
-  const std::shared_ptr<DownstreamAddrGroup> &group_;
+  std::shared_ptr<DownstreamAddrGroup> group_;
   // Address of remote endpoint
   DownstreamAddr *addr_;
   IOControl ioctrl_;