The upcoming new implementation of attach_xmldoc needs HeapAlloc. Prepare
for the failure case.

This->node should never be zero. Each IXMLDOMNode (and derived interfaces)
always represents a present libxml data object. There is no no-data
IXMLDOMNode, although there might be documents without a root element.

attach_xmlnode is only used to attach XML document nodes, and a cast
is needed on each call. Rename it to attach_xmldoc, change the
parameter type to move the cast at one point into the function.

The FIXME messages for unknown interfaces should not be emitted for
interfaces that should not get implemented. Origin 8 spams you with
these messsages.

The libxml documentation suggests to either use
xmlEncodedEntitiesReentrant or xmlEncodeSpecialChars, but if you apply
both, you end up producing "&".