sw: valgrind complains about uninitialized mbIsPrepareSelAll

Change-Id: I5405a6818fc63c3e29bb189858fd0abc73f67e43
(cherry picked from commit c6f3d909f8e5c64b6bc36a13dfd0d46eb0e1d88d)
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
(cherry picked from commit 4c9ddd594e26c517b8515f77206bfeb2fb063e03)