Ignore tcp sent cb when there's no asscociated nc

TCP ACK may arrive after the we've corresponding mg_connection has already been closed.
Fixes cesanta/mongoose-os#235

PUBLISHED_FROM=8164b86131ef08c1b2a6a08630920d600b3f91ac

mongoose.c

@@ -14203,6 +14203,7 @@ static err_t mg_lwip_tcp_sent_cb(void *arg, struct tcp_pcb *tpcb,
                                  u16_t num_sent) {
   struct mg_connection *nc = (struct mg_connection *) arg;
   DBG(("%p %p %u", nc, tpcb, num_sent));
+  if (nc == NULL) return ERR_OK;
   if ((nc->flags & MG_F_SEND_AND_CLOSE) && !(nc->flags & MG_F_WANT_WRITE) &&
       nc->send_mbuf.len == 0 && tpcb->unacked == 0) {
     mg_lwip_post_signal(MG_SIG_CLOSE_CONN, nc);