Invalid Memory Access due to asynchronous loading in QQuickImage.

In the QQuickImage object when a new image is loaded asynchronously (via http) it is possible for the old image to be released from memory but still remain referenced in the renderer. If the render thread runs between the time the old image was released and before the new image is loaded there will be an invalid memory access when trying to render the released texture.

I have attached a patch that appears to 'fix' the problem but I can not vouch for the validity of the fix.

The attached qml file will reproduce the problem. I did not attach the image files I was using in the test. The image files were located on a separate machine for the test.