selecting and deselecting text from start to end crash in QTextEngine:shape

since this patch QTBUG-70946

my test application crash

I didn't test this on other OSs so just Windows

Step to reproduce:

1.open app

2.start selecting text from beginning to end with mouse cursor and then move cursor to start of the text

3.repeat step to again and again

4.and one time it crash

It will crash http://code.qt.io/cgit/qt/qtbase.git/tree/src/gui/text/qtextengine.cpp?h=5.12.1#n1982

call stack from visual studio and some local values

I build qt 5.12.1

git clone https://code.qt.io/qt/qt5.git

cd qt5

git fetch --all --tags --prune

git checkout 5.12.1

git pull

git submodule update --init

git submodule update

configure.bat -no-directwrite -no-direct2d -no-opengles3 -no-dbus -no-sql-odbc -no-sql-psql -qt-zlib -qt-libpng -qt-libjpeg -no-sse4.1 -no-sse4.2 -no-avx -no-avx2 -nomake examples -nomake tests -skip qt3d -skip qtactiveqt -skip qtcanvas3d -skip qtcharts -skip qtdatavis3d -skip qtfeedback -skip qtgamepad  -skip qtquickcontrols2 -skip qtcharts -skip qtlocation -skip qtmacextras -skip qtnetworkauth -skip qtpim -skip qtpurchasing -skip qtremoteobjects -skip qtsensors -skip qtserialbus -skip qtserialport -skip qtspeech -skip qtvirtualkeyboard -skip qtwayland -skip qtwebengine -skip qtwebchannel -skip qtwebsockets -skip qtwebview -skip qtx11extras -skip qttools -skip qtsystems -skip qtconnectivity -skip qtwinextras -skip qtmultimedia -skip qtxmlpatterns  -qt-harfbuzz -qt-freetype -platform win32-msvc -opensource -v -confirm-license -developer-build -opengl desktop -no-feature-cups -no-feature-lcdnumber -no-feature-ftp -no-feature-d3d12 -no-feature-qml-debug -no-qml-debug -debug -force-debug-info