Don't crash when SwitchPanelImages is None.
The whole changeImage() function in switchpanel.c is a no-op if
wPreferences.swtileImage is set to None because the panel isn't actually
drawn in that case. As a consequence there are no images to change.
As well as being logically incorrect the existing code causes a segfault
if the user has the SwitchPanelImages preference set to None because
changeImage() would attempt to access the icons and images arrays which
are only initialised in wInitSwitchPanel() when swtileImage has a value.
Bug report and diagnosis by Juan Giordana, Gabriel Vlasiu and Christophe
Curis.