extmod/modlwip: Register TCP close-timeout callback before closing PCB.

In d5f0c87b this call to tcp_poll() was
added to put a timeout on closing TCP sockets.  But after calling
tcp_close() the PCB may be freed and therefore invalid, so tcp_poll() can
not be used at that point.  As a fix this commit calls tcp_poll() before
closing the TCP PCB.  If the PCB is subsequently closed and freed by
tcp_close() or tcp_abort() then the PCB will not be on any active list and
the callback will not be executed, which is the desired behaviour (the
_lwip_tcp_close_poll() callback only needs to be called if the PCB remains
active for longer than the timeout).

extmod/modlwip.c

@@ -1421,12 +1421,15 @@ STATIC mp_uint_t lwip_socket_ioctl(mp_obj_t self_in, mp_uint_t request, uintptr_
 
         switch (socket->type) {
             case MOD_NETWORK_SOCK_STREAM: {
+                if (socket->pcb.tcp->state != LISTEN) {
+                    // Schedule a callback to abort the connection if it's not cleanly closed after
+                    // the given timeout.  The callback must be set before calling tcp_close since
+                    // the latter may free the pcb; if it doesn't then the callback will be active.
+                    tcp_poll(socket->pcb.tcp, _lwip_tcp_close_poll, MICROPY_PY_LWIP_TCP_CLOSE_TIMEOUT_MS / 500);
+                }
                 if (tcp_close(socket->pcb.tcp) != ERR_OK) {
                     DEBUG_printf("lwip_close: had to call tcp_abort()\n");
                     tcp_abort(socket->pcb.tcp);
-                } else {
-                    // If connection not cleanly closed after timeout then abort the connection
-                    tcp_poll(socket->pcb.tcp, _lwip_tcp_close_poll, MICROPY_PY_LWIP_TCP_CLOSE_TIMEOUT_MS / 500);
                 }
                 break;
             }