Prepare for upcoming compile breakage

Qt 5.9 is going to break source compatibility by making the
QStringRef(QString *) constructor explicit.

Since the use of that constructor rather looks unintentional
here, replace its use.

Change-Id: I6f5f2e79f7d92f6e80a1ea0aa12126d9c88b499e
Reviewed-by: Jochen Becher <jochen_becher@gmx.de>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>