Fix livelock at application exit if threads were running

This only happened in debug mode, though, because in release mode the
warning wasn't printed and the socket notifier was removed. In debug
mode, this loop in closingDown() never exited:

    while (!d->sn_read.isEmpty())
        unregisterSocketNotifier((*(d->sn_read.begin()))->obj);

[ChangeLog][QtCore][QThread] Fixed a bug that would cause debug-mode
applications to live lock on exit if they had a global static containing
a QThread that wasn't properly exited.

Task-number: QTBUG-49870
Change-Id: I7a9e11d7b64a4cc78e24ffff142e457a4540d6b6
Reviewed-by: Mat Sutcliffe <oktal3700@gmail.com>
Reviewed-by: Roland Winklmeier <Roland.M.Winklmeier@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>