QTextEngine crashes when navigating over a line that contains a trailing tab character

Because of https://codereview.qt-project.org/244594, qtextengine.cpp crashes when pressing Key_Down (or Key_Up etc.) in a QTextEdit that contains a trailing tab character.

#2  <signal handler called>
#3  0x00007fde299a2942 in QTextEngine::shape (this=0x16a5ad50, item=item@entry=3) at /d/qt/5/kde/qtbase/src/gui/text/qtextengine.cpp:1982
#4  0x00007fde299b27b9 in QTextLine::xToCursor (this=this@entry=0x7fffe00fecd0, _x=<optimized out>, cpos=cpos@entry=QTextLine::CursorBetweenCharacters) at /d/qt/5/kde/qtbase/src/gui/text/qtextlayout.cpp:3047
#5  0x00007fde299ffa05 in QTextCursorPrivate::movePosition (this=0x16f1c010, op=op@entry=QTextCursor::Down, mode=mode@entry=QTextCursor::MoveAnchor) at /d/qt/5/kde/qtbase/src/gui/text/qtextcursor.cpp:587
#6  0x00007fde29a0245e in QTextCursor::movePosition (this=this@entry=0x16bf0ca0, op=op@entry=QTextCursor::Down, mode=mode@entry=QTextCursor::MoveAnchor, n=n@entry=1) at /d/qt/5/kde/qtbase/src/gui/text/qtextcursor.cpp:1253
#7  0x00007fde2a2cb9eb in QWidgetTextControlPrivate::cursorMoveKeyEvent (this=this@entry=0x16bf0c20, e=e@entry=0x7fffe00ffda0) at /d/qt/5/kde/qtbase/src/widgets/widgets/qwidgettextcontrol.cpp:281
#8  0x00007fde2a2cda9a in QWidgetTextControlPrivate::keyPressEvent (this=this@entry=0x16bf0c20, e=e@entry=0x7fffe00ffda0) at /d/qt/5/kde/qtbase/src/widgets/widgets/qwidgettextcontrol.cpp:1217
#9  0x00007fde2a2d0a39 in QWidgetTextControl::processEvent (this=<optimized out>, e=0x7fffe00ffda0, matrix=..., contextWidget=0x16d26e00) at /d/qt/5/kde/qtbase/src/widgets/widgets/qwidgettextcontrol.cpp:1023
#10 0x00007fde2a2c789a in QWidgetTextControl::processEvent (this=this@entry=0x16d2b260, e=e@entry=0x7fffe00ffda0, coordinateOffset=..., contextWidget=contextWidget@entry=0x16d26e00) at /d/qt/5/kde/qtbase/src/widgets/widgets/qwidgettextcontrol.cpp:983

Testcase:

cd qtbase/examples/widgets/richtext/textedit
wget http://www.davidfaure.fr/2018/mytext
./textedit mytext

and then press Key_Down 4 times.

The new code says

ushort *lc = logClusters(&li);
*lc = item ? lc[-1] : 0;

and lc is nullptr (I added Q_ASSERT(lc) and it hits that assert).

Is it enough to put an if(lc) around all this, i.e. do nothing if lc is null?