Resolves: fdo#82355 crash on loading ppt

regression probably triggered by...

commit 2e3c45af
Date:   Fri Mar 4 07:38:16 2011 -0800
Remove usage of deprecated List container.

but that just stopped at the first NULL by chance

Change-Id: I767e3bc3130d99cedceb4172857d1741c677b19b

sd/source/core/sdpage2.cxx

@@ -208,9 +208,10 @@ void SdPage::SetPresentationLayout(const OUString& rLayoutName,
 
                 if (pSheet != pOldSheet)
                 {
-                    pObj->EndListening(*pOldSheet);
+                    if (pOldSheet)
+                        pObj->EndListening(*pOldSheet);
 
-                    if (!pObj->IsListening(*pSheet))
+                    if (pSheet && !pObj->IsListening(*pSheet))
                         pObj->StartListening(*pSheet);
                 }