Fix a potential null pointer acceess issue. #1854

It's possible that c-ares calls the callback function immediately in ares_gethostbyname, which may free the server
structure in that callback function.

src/server.c

@@ -903,6 +903,8 @@ server_recv_cb(EV_P_ ev_io *w, int revents)
                 ev_io_start(EV_A_ & remote->send_ctx->io);
             }
         } else {
+            ev_io_stop(EV_A_ & server_recv_ctx->io);
+
             query_t *query = ss_malloc(sizeof(query_t));
             memset(query, 0, sizeof(query_t));
             query->server = server;
@@ -920,8 +922,6 @@ server_recv_cb(EV_P_ ev_io *w, int revents)
                 close_and_free_server(EV_A_ server);
                 return;
             }
-
-            ev_io_stop(EV_A_ & server_recv_ctx->io);
         }
 
         return;