Use the stateless UTF-8 encoder in QStringRef::toUtf8

QString::toUtf8 already does it. I guess I forgot to update this part in
d51130cc3a00df8147e2eb0799e06865c901c6e0.

Change-Id: I83feafcb0383758f7e64d5142f57a7ae6a2ff351
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>