regression since aa1927dc

 mpViewShell(pViewShell),
+mpSlideSorterViewShell(
          ::boost::dynamic_pointer_cast< ::sd::slidesorter::SlideSorterViewShell >( pViewShell )),

both are boost::shared_ptr potentially to the same thing then, given that we
reset mpViewShell in ::disposing we presumably have to do the same thing to
mpSlideSorterViewShell.

add unset check on mpSlideSorterViewShell usages while I'm at it

Change-Id: I35f51977c40f946a49789abd9405a4ef84aea0e8