[winswitch] xpra crash on exit

[Douglas Doole]

My system currently has xpra 0.15.3 on the client and 0.15.2 on the server
(both running Ubuntu 14.04). When I exited the client today, it crashed
with the following output:

2015-07-15 07:06:30,708 disconnect_and_quit(0, client exit)
2015-07-15 07:06:30,709 disconnect_and_quit: protocol_closed()
2015-07-15 07:06:30,710 UIXpraClient.cleanup()
2015-07-15 07:06:30,710 XpraClientBase.cleanup() protocol=Protocol(None)
2015-07-15 07:06:30,710 UIXpraClient.cleanup() calling <class
'xpra.client.gtk_base.gtk_keyboard_helper.GTKKeyboardHelper'>.cleanup()
2015-07-15 07:06:30,711 UIXpraClient.cleanup() calling <class
'xpra.clipboard.gdk_clipboard.GDKClipboardProtocolHelper'>.cleanup()
2015-07-15 07:06:30,712 UIXpraClient.cleanup() calling <class
'xpra.client.gtk_base.statusicon_tray.GTKStatusIconTray'>.cleanup()
2015-07-15 07:06:30,712 UIXpraClient.cleanup() calling <class
'xpra.client.notifications.dbus_notifier.DBUS_Notifier'>.cleanup()
2015-07-15 07:06:30,712 UIXpraClient.cleanup() calling <class
'xpra.client.gtk2.tray_menu.GTK2TrayMenu'>.cleanup()
2015-07-15 07:06:30,713 UIXpraClient.cleanup() calling <class
'xpra.platform.xposix.gui.ClientExtras'>.cleanup()
Xpra: Fatal IO error 11 (Resource temporarily unavailable) on X server :0.
[xcb] Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not
been called
[xcb] Aborting, sorry about that.
python: ../../src/xcb_io.c:274: poll_for_event: Assertion
`!xcb_xlib_threads_sequence_lost' failed.
Aborted (core dumped)

This was a one time crash. I have used this combination of client/server
before and after the crash successfully.