Invalidate old QImage data if load()/loadFromData() has failed

Invalidate old QImage data if load()/loadFromData() has failed

This guarantees one will never get `!img.isNull()` after
load()/loadFromData() has failed, even if the image was
not null before.
Apply the same fix to QPixmap and QPicture.

Change-Id: Ida1ad6a6f0fc830df8e75ada0c163fc2d3360dea
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>