Fix potential memory corruption.

Fix potential memory corruption.

The code exploited information that most of types in union use d
pointers and that size of d pointer is less or equals then
sizeof(QVariant) or sizeof(double). Still the code may suffer
from an alignment issue on some exotic platforms.

Change-Id: I4ef331f4cdb7177337ddcc8696f78d85e9594d27
Reviewed-on: http://codereview.qt-project.org/4244
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>