Resolves: tdf#91060 don't crash on loading certain pptx

Change-Id: Ibf32ef644222a59c5fe6a9736ff64739ab24b74f
(cherry picked from commit c874bf3f)
Reviewed-on: https://gerrit.libreoffice.org/18225Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>

oox/source/ppt/slidefragmenthandler.cxx

@@ -194,7 +194,7 @@ SlideFragmentHandler::~SlideFragmentHandler()
     case PPT_TOKEN( cmLst ):
         break;
     case PPT_TOKEN( cm ):
-        if(!mpSlidePersistPtr->getCommentsList().cmLst.empty())
+        if (!mpSlidePersistPtr->getCommentsList().cmLst.empty() && !getCharVector().empty())
         {
             // set comment text for earlier comment
             mpSlidePersistPtr->getCommentsList().cmLst.back().setText( getCharVector().back() );