Also avoids needlessly initializing static variables.

Some apps depend on this quirk because they don't reset the index
before subsequent calls to HttpQueryInfo.

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.