QIMAGE_SANITYCHECK_MEMORY is missing in some methods

In many of the functions in QImage where a new QImage is created,
QIMAGE_SANITYCHECK_MEMORY(image) is called afterwards to check if it is a null image.

In
QImage QImage::rgbSwapped() and QImage::mirrored()
QIMAGE_SANITYCHECK_MEMORY()
is not called after a new QImage is created.

When some functions return a null image, for example when there is not enough memory to create the image it is easy to get an exception or a crash.