Fix memory leak in SvxMSDffManager::ImportShape.

When a new DffPropertyReader is created and assigned to pSecPropSet
make sure the old one is deleted first.

Change-Id: Idd14fdf4e3a03a625a10a89dde71ad66cbdba792
Reviewed-on: https://gerrit.libreoffice.org/4761Reviewed-by: Petr Mladek <pmladek@suse.cz>
Tested-by: Petr Mladek <pmladek@suse.cz>

filter/source/msfilter/msdffimp.cxx

@@ -4141,6 +4141,7 @@ SdrObject* SvxMSDffManager::ImportShape( const DffRecordHeader& rHd, SvStream& r
     if ( aObjData.bOpt2 )
     {
         maShapeRecords.Current()->SeekToBegOfRecord( rSt );
+        delete pSecPropSet;
         pSecPropSet = new DffPropertyReader( *this );
         pSecPropSet->ReadPropSet( rSt, NULL );
     }