QTextEngine crashes for some characters

QTextEngine crashes for some characters (ones not common in fonts) in case when fallback font families list consist of more than 255 entries.

Could be reproduced with the following code:
for(int b1 = 10;b1 < 300;b1++)

{ QString myStr; myStr.append(QChar(ushort(b1))); ui->comboBox->addItem(myStr); }

This causes SIGSEGV in QtextEngine::shapeTextWithHarfbuzz() at line
"si.ascent = qMax(actualFontEngine->ascent(), si.ascent);"
due to actualFontEngine==0.