Qt may call ucnv_getDefaultName() without calling setlocale() first

When using ICU, Qt may call ucnv_getDefaultName() without calling setlocale(.., "") first, which is required before calling ucnv_getDefaultName().
One such possible path is:
QString::fromLocal8Bit() -> QTextCodec::codecForLocale() -> QIcuCodec::defaultCodecUnlocked() -> ucnv_getDefaultName()

setlocale() is called in QCoreApplicationPrivate::initLocale(), which may not have been called.
A call to initLocale() should be added to QIcuCodec::defaultCodecUnlocked(). When not using ICU, initLocale() is called in setupLocaleMapper(), so it should also be done when using ICU, or the call can be moved into QTextCodec::codecForLocale().

A real problem appears when using Gwenview 16.11.90 from KDE5 on FreeBSD with locale set to zh_CN.UTF-8. It cannot display image files which have Chinese characters in their paths.