Check newconn, not conn, before marking newconn for close

This fixes bug 2757.  It's a bugfix on 85da676108.

src/or/connection.c

@@ -1178,7 +1178,7 @@ connection_handle_listener_read(connection_t *conn, int new_type)
   }
 
   if (connection_init_accepted_conn(newconn, conn->type) < 0) {
-    if (! conn->marked_for_close)
+    if (! newconn->marked_for_close)
       connection_mark_for_close(newconn);
     return 0;
   }