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>

src/widgets/widgets/qplaintextedit.cpp

@@ -839,7 +839,8 @@ void QPlainTextEditPrivate::_q_textChanged()
 
     placeholderVisible = !placeholderText.isEmpty()
             && q->document()->isEmpty()
-            && q->firstVisibleBlock().layout()->preeditAreaText().isEmpty();
+            && (!q->firstVisibleBlock().isValid() ||
+                 q->firstVisibleBlock().layout()->preeditAreaText().isEmpty());
 
     if (placeholderCurrentyVisible != placeholderVisible)
         viewport->update();