Fixes #333.
I cannot reproduce this crash, but looks like the crash happens during
exit, I am thinking perhaps the buddylist window gets closed before the
plugin tries to remove the statusboxes.
/*
* irssi - Implements several irssi features for Purple
* Copyright (C) 2005-2008 Gary Kramlich <grim@reaperworld.com>
* Copyright (C) 2006-2008 John Bailey <rekkanoryo@rekkanoryo.org>