qpa: Fix memory leak of QFontEngineBox object

Delete QFontEngineBox object in QFontDatabase::load() if the object can not
be used.

cherry-picked from qt5/qtbase effbc9edc57fd8a2e40729a76004fe4f5fafcf49
original patch from jianliang79 <jianliang79@gmail.com>

Modified to match changes in QAtomicInt API.

Change-Id: I1cf7398f582c918426a73973347efe4425100eec
Reviewed-by: jian liang <jianliang79@gmail.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Julien Brianceau <jbrianceau@nds.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>