pClientData can be null

Change-Id: I9375f84affdaef30d9b1ded31525e6b7b8709cbc
(cherry picked from commit 7d4c77e3)
Reviewed-on: https://gerrit.libreoffice.org/18149Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>

filter/source/msfilter/svdfppt.cxx

@@ -2587,7 +2587,7 @@ bool SdrPowerPointImport::GetColorFromPalette( sal_uInt16 nNum, Color& rColor )
 bool SdrPowerPointImport::SeekToShape( SvStream& rSt, void* pClientData, sal_uInt32 nId ) const
 {
     bool bRet = SvxMSDffManager::SeekToShape( rSt, pClientData, nId );
-    if ( !bRet )
+    if (!bRet && pClientData)
     {
         ProcessData& rData = *static_cast<ProcessData*>(pClientData);
         PptSlidePersistEntry& rPersistEntry = rData.rPersistEntry;