SwAccessibleFrameBase::Modify(): avoid accessing deleted mpFrm

In ~SwDoc when a SwFmt is deleted,
SwAccessibleNoTextFrame::GetNoTxtNode() would access the mpFrm which
at that point was already deleted too; avoid that by returning early.

This can be reproduced by closing the bugdoc from fdo#71450.

Change-Id: Ide4d42511ccad116fa0362c23ced299d8430d08a

sw/source/core/access/accnotextframe.cxx

@@ -98,6 +98,8 @@ void SwAccessibleNoTextFrame::Modify( const SfxPoolItem* pOld, const SfxPoolItem
          msTitle.isEmpty() )
     {
         SwAccessibleFrameBase::Modify( pOld, pNew );
+        if (!GetRegisteredIn())
+            return; // probably was deleted - avoid doing anything
     }
 
     const SwNoTxtNode *pNd = GetNoTxtNode();