Memory leaks in QAlphaWidget and QRollEffect

The static variables q_blend and q_roll are set to null without calling deleteLater() first when they are not already null.

We have fixed this locally with these changes.

In QAlphaWidget::render() replace:
  q_blend = 0;
and:
  q_blend = nullptr
with:
  qFadeEffect( nullptr );

In QRollEffect::scroll() replace:
  q_roll = nullptr;
with:
  qScrollEffect( nullptr );

This bug was discovered in Qt 6.5.1 but also exists in the latest version of the code.