Invalid assumption (and assertion) in QLineF::setLength

https://codereview.qt-project.org/q/I7b71d66b872ccc08a64e941acd36b45b0ea15fab added a Q_ASSERT in QLineF::setLength that can be hit using this simple test:

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	QLineF line(QPointF(qQNaN(), qQNaN()), QPointF(qQNaN(), qQNaN()));
	line.setLength(9.45);

	return a.exec();
}

In this situation isValid() returns true for the line, but length() is NaN, which is not >0, so the Q_ASSERT is hit.

Our application, which has its own programming language, allows users to draw lines, rectangles, etc. programmatically. We have a suite of fuzz tests to test this functionality, and one of the tests is now hitting this Q_ASSERT.

We are also noticing a hang in the same fuzz tests when drawing rectangles that did not happen when we were using version 5.12.9. I haven't figured out a MCE to reproduce that hang yet, but it is almost certainly caused by the same Qt change as this bug.

Clearly there is no realistic use case for drawing a line between to (Nan, Nan) points, but if the code that calls QLineF and such is now responsible for checking the validity of the values, that's a significant enough change that I would at least expect a change log entry.