socket/win32: flush pending read before signaling HUP

Unix and Windows gio GSocket behaves differently when the socket is
closed by the peer. On Unix, the client receives pending data before
receiving HUP. But on Windows, the HUP may come before, resulting in
unreliable and racy code. We should have same behaviour on all
platforms.

According to MSDN documentation: "an application should check for
remaining data upon receipt of FD_CLOSE to avoid any possibility of
losing data."

https://bugzilla.gnome.org/show_bug.cgi?id=669810