msxml3: get_attributes not supported for Attribute, CData, Comment, Doc, Doc Frag, Entity and Text Nodes.

First of all, the code was freeing the wrong pointer (i.e. the pointer
supplied by the caller of one of the NdrFullPointer* functions, not the
PFULL_PTR_TO_REFID_ELEMENT. Second, the code wasn't following the Next
link to the next entry in the list.

Add tests for this function.