fixup! Attr: (internal) added move method

This is a severe bug. The purpose of Attr_free is to free the internal
data, while leaving the outer structure unfreed. What is needed here is
quite the opposite: freeing the outer structure but leaving the internal
data intact. Of course the internal data is still needed, as it is data
that was just assigned to the dest Attr. This bug introduced a double
free, which is undefined behaviour with all of it's scary effects.