Resolves: fdo#55974 segfault while closing an Impress file

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

sd/source/ui/framework/factories/ViewShellWrapper.cxx

@@ -95,6 +95,7 @@ void SAL_CALL ViewShellWrapper::disposing (void)
         xWindow->removeWindowListener(this);
     }
 
+    mpSlideSorterViewShell.reset();
     mpViewShell.reset();
 }
 
@@ -145,6 +146,9 @@ sal_Bool SAL_CALL ViewShellWrapper::isAnchorOnly (void)
 
 sal_Bool SAL_CALL ViewShellWrapper::select( const ::com::sun::star::uno::Any& aSelection ) throw(lang::IllegalArgumentException, uno::RuntimeException)
 {
+    if (!mpSlideSorterViewShell)
+        return false;
+
     bool bOk = true;
 
     ::sd::slidesorter::controller::SlideSorterController& rSlideSorterController
@@ -180,6 +184,9 @@ uno::Any SAL_CALL ViewShellWrapper::getSelection() throw(uno::RuntimeException)
 {
     Any aResult;
 
+    if (!mpSlideSorterViewShell)
+        return aResult;
+
     slidesorter::model::PageEnumeration aSelectedPages (
         slidesorter::model::PageEnumerationProvider::CreateSelectedPagesEnumeration(
             mpSlideSorterViewShell->GetSlideSorter().GetModel()));