extmod/modlwip: Free any incoming bufs/connections before closing PCB.

Commit 2848a613 introduced a bug where
lwip_socket_free_incoming() accessed pcb.tcp->state after the PCB was
closed.  The state may have changed due to that close call, or the PCB may
be freed and therefore invalid.  This commit fixes that by calling
lwip_socket_free_incoming() before the PCB is closed.