Update libxml2 to 2.9.4.

Add license block.
Pull extra patch to fix NULL pointer deref. [1]

Changelog: https://mail.gnome.org/archives/xml/2016-May/msg00023.html

PR: 209806
Submitted by: pi@
Obtained from: upstream [1]
MFH: 2016Q3
Security: e195679d-045b-4953-bb33-be0073ba2ac6