Regression: Random crashs in shapeTextWithHarfbuzzNG with multi threaded font usage

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff25a2700 (LWP 15673)]
0x00007ffff6fc2515 in ?? () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5
(gdb) bt
#0 0x00007ffff6fc2515 in ?? () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5
#1 0x00007ffff6faa394 in ?? () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5
#2 0x00007ffff6fa9dde in ?? () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5
#3 0x00007ffff6d9fa77 in QTextEngine::shapeTextWithHarfbuzzNG(QScriptItem const&, unsigned short const*, int, QFontEngine*, QVector<unsigned int> const&, bool) const () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5
#4 0x00007ffff6da352b in QTextEngine::shapeText(int) const () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5
#5 0x00007ffff6da41db in QTextEngine::shape(int) const () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5
#6 0x00007ffff6db7cd7 in QTextLine::layout_helper(int) () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5
#7 0x00007ffff6db8b1b in QTextLine::setLineWidth(double) () from /local/cullmann/build/aisee4.default.O0/usr/bin/libQt5Gui.so.5