SwAccessibleFrameBase::Modify(): avoid accessing deleted mpFrm
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

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core