libxml2: fix CVE-2016-4658 Disallow namespace nodes in XPointer points and ranges

libxml2: fix CVE-2016-4658 Disallow namespace nodes in XPointer points and ranges

Namespace nodes must be copied to avoid use-after-free errors.
But they don't necessarily have a physical representation in a
document, so simply disallow them in XPointer ranges.

Signed-off-by: Andrej Valek <andrej.valek@siemens.com>
Signed-off-by: Pascal Bach <pascal.bach@siemens.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>