When traversing the "preceding" axis from an attribute node, we must
first go up to the attribute's containing element. Otherwise, text
children of other attributes could be returned. This made it possible
to hit a code path in xmlXPathNextAncestor which contained another bug:
The attribute node was initialized with the context node instead of the
current node. Normally, this code path is only hit via
xmlXPathNextAncestorOrSelf in which case the current and context node
are the same.
The combination of the two bugs could result in an infinite loop, found
with libFuzzer.
Traversing the "following" and the "preceding" axis from namespace nodes
should be handled similarly. This wasn't supported at all previously.
If the context node is an attribute, the attribute itself is on the
descendant-or-self axis. The principal node type of this axis is element,
so the only node test that can return the attribute is "node()". In other
words, "@attr/descendant-or-self::node()" is equivalent to "@attr".
This matches the behavior of Saxon-CE.
