extmod/modlwip: Still process remaining incoming data of a closed socket.

It can happen that a socket gets closed while the pbuf is not completely
drained by the application.  It can also happen that a new pbuf comes in
via the recv callback, and then a "peer closed" event comes via the same
callback (pbuf=NULL) before the previous event has been handled.  In both
cases the socket is closed but there is remaining data.  This patch makes
sure such data is passed to the application.

extmod/modlwip.c

@@ -384,11 +384,6 @@ STATIC mp_uint_t lwip_tcp_send(lwip_socket_obj_t *socket, const byte *buf, mp_ui
 
 // Helper function for recv/recvfrom to handle TCP packets
 STATIC mp_uint_t lwip_tcp_receive(lwip_socket_obj_t *socket, byte *buf, mp_uint_t len, int *_errno) {
-
-    if (socket->state == STATE_PEER_CLOSED) {
-        return 0;
-    }
-
     if (socket->incoming.pbuf == NULL) {
         mp_uint_t start = mp_hal_ticks_ms();
         while (socket->state == STATE_CONNECTED && socket->incoming.pbuf == NULL) {
@@ -399,7 +394,10 @@ STATIC mp_uint_t lwip_tcp_receive(lwip_socket_obj_t *socket, byte *buf, mp_uint_
             poll_sockets();
         }
         if (socket->state == STATE_PEER_CLOSED) {
-            return 0;
+            if (socket->incoming.pbuf == NULL) {
+                // socket closed and no data left in buffer
+                return 0;
+            }
         } else if (socket->state != STATE_CONNECTED) {
             *_errno = -socket->state;
             return -1;