Null icons handling changed in 4.6.0

There is a little code snippet:

#include <QApplication>
#include <QIcon>
#include <QDebug>
//
int main(int argc, char *argv[]) {
  QApplication app(argc, argv);

  qDebug() << QIcon().isNull();
  qDebug() << QIcon().pixmap(16, 16).size();
}

Using Qt 4.5.3 it prints:

true
QSize(0, 0)

And with Qt 4.6.0:

true
QSize(-1, -1)

In fact, on Linux it affects QMdiSubWindow class when no custom icon is set for the application.
On each instance creation I get the following warning:

QWidget::setMinimumSize: (/QMdi::ControlLabel) Negative sizes (-1,-1) are not possible

This is because inner class ControlLabel is trying to set it's minimum size to the size of a pixmap converted from QApplication::windowIcon() which is null.