1、更正lwip_accept()中调用netconn_peer()失败时，netconn_delete()会被调用两次，从而导致进入rt_assert_handler()错误。
原因：(https://savannah.nongnu.org/bugs/?55536) bug #55536: lwIP 2.1.2: netconn_delete() called twice from lwip_accept()