toTimeZone() is segfaulting when called with an invalid time zone

Such call is segfaulting `.toTimeZone(QTimeZone())`. Indeed, looking at the code, it is making the date time invalid in `::setTimeZone()`, but it is calling `::fromMSecsSinceEpoch()` anyway, where line 4166, it is acessing internal of the `m_timeZone` object without checking that this internal exists.