Fix xsltNumberFormatGetMultipleLevel

Namespace nodes are actually an xmlNs, not an xmlNode. They must be
special-cased in xsltNumberFormatGetMultipleLevel to avoid an
out-of-bounds heap access.

Move the test whether a node matches the "count" pattern to a separate
function to make the code more readable. As a side effect, we also
compare expanded names when walking up the ancestor axis, fixing an
insignificant bug.