QSharedDataPointer doesn't destroy old data on move assignment

The current move assignment operator implementation for QSharedDataPointer and QExplicitlySharedDataPointer is a simple swap of the left hand side and right hand side data pointers. I think this is not sufficient since the right hand side pointer isn't cleared afterwards.

When moving a reference from a shared pointer `p2` to `p1` like this

p1 = std::move(p2)

the old reference in `p1` is expected to be gone, possibly invoking the data destructor of the object `p1` was referring to. Due to the swap however the old `p1` data now lives on in `p2` and is destroyed on some later, unexpected occasion (or maybe even not at all).