Fix memory leak in QFileInfo::exists()

Use the the legacy file engine object created in static function
QFileInfo::exists() as the engine of the QFileInfo object to prevent
memory leak. This can also boost a little performance.

Change-Id: I06317d158d487be5ef15fe3244a917a371563ac9
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>