QPainter: Fix a hard to trigger null dereference in QPainterPrivate::drawTextItem.

Done-with: Mohammed Hassan <mohammed.hassan@jollamobile.com>
Change-Id: I24c724d24346fb5be8c4a66d68a11d51be5ad5f2
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>