socket: set fd field to -1 after closing socket

This ensures that g_socket_get_fd() will return -1 after the socket has
been closed.

https://bugzilla.gnome.org/show_bug.cgi?id=765959

gio/gsocket.c

@@ -3285,6 +3285,7 @@ g_socket_close (GSocket  *socket,
       break;
     }
 
+  socket->priv->fd = -1;
   socket->priv->connected_read = FALSE;
   socket->priv->connected_write = FALSE;
   socket->priv->closed = TRUE;