don't access freed memory

Change-Id: I3839d9af55c7dcd29d34850618a33c6c3b1ee2ec

sc/source/core/data/postit.cxx

@@ -845,6 +845,9 @@ ScPostIt* ScNoteUtil::CreateNoteFromCaption(
         // ScNoteCaptionCreator c'tor updates the caption object to be part of a note
         ScNoteCaptionCreator aCreator( rDoc, rPos, rCaption, bShown );
     }
+    else
+        return NULL;
+
     return pNote;
 }