coverity#704642 Dereference after null check

Change-Id: Iae9b7caf2076722a01e6882921622b193f2b4349

filter/source/msfilter/svdfppt.cxx

@@ -2537,6 +2537,9 @@ bool SdrPowerPointImport::GetColorFromPalette( sal_uInt16 nNum, Color& rColor )
                         else
                             pMasterPersist = (*pPageList2)[ nNextMaster ];
                     }
+                }
+                if ( pMasterPersist )
+                {
                     ((SdrPowerPointImport*)this)->aPageColors = pMasterPersist->aColorScheme;
                 }
             }