The xmlXPathNext* iterators rely on the XPath context node being set to
the start node of the iteration. Some parts of the code base like the
xsl:key functions also leave the context node in an unspecified state.
Make sure that the context node is reset before invoking the XPath
iterators. Also backup and restore the context node in
xsltNumberFormatGetMultipleLevel for good measure.

This bug could also lead to type confusion and invalid reads in
connection with namespace nodes.

Fixes #13. Also see the Chromium bug report:

https://bugs.chromium.org/p/chromium/issues/detail?id=930663

Thanks to Nicolas Grégoire for the report.