TCP: check for an active connection before cleaning it up

If there is no active connection (e.g. it is waiting in accept), then
the connection object, which doesn't yet exist, should not be cleaned
up when the socket is closed.

net/tcp/tcp_send_buffered.c

@@ -507,9 +507,12 @@ static uint16_t psock_send_interrupt(FAR struct net_driver_s *dev,
     {
       nllvdbg("Lost connection: %04x\n", flags);
 
-      /* Report not connected */
+      if (psock->s_conn != NULL)
+        {
+          /* Report not connected */
 
-      net_lostconnection(psock, flags);
+          net_lostconnection(psock, flags);
+        }
 
       /* Free write buffers and terminate polling */