QPlainTextEdit: make sure firstVisibleBlock() is valid

Under some circumstances it's possible that firstVisibleBlock() returns
an invalid block within QPlainTextEditPrivate::_q_textChanged() which
results in a nullptr access later on.
Therefore add a check similar to other places and test the validity of
the returned block before accessing it.

Fixes: QTBUG-80929
Change-Id: I1fd4643b10b842acfe1c356048379f0ba225dddf
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

0 files changed, 0 insertions, 0 deletions