xfce4-settings: fix crash when no pointer device is connected

* reported by Cristian Prigoana
* updated [1]

[1] https://bugzilla.xfce.org/show_bug.cgi?id=9474



Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>