libxml2: security bump to version 2.9.4

libxml2: security bump to version 2.9.4

Fixes a bunch of security issues including:

  CVE-2016-1762: Heap-based buffer overread in xmlNextChar

  CVE-2016-1834: heap-buffer-overflow in xmlStrncat

  CVE-2016-3705: Missing increments of recursion depth counter to XML parser

A few more security fixes are listed in the release announcement at
https://mail.gnome.org/archives/xml/2016-May/msg00023.html.

Also fixes:
http://autobuild.buildroot.net/results/6db/6db405a097b192876c0b1b8d59051d614563c617/
http://autobuild.buildroot.net/results/62a/62addf4abd2a0df8222a81a83c16b2b9a61c9481/
http://autobuild.buildroot.net/results/204/20402690ad05d10d456a219da5252a38badf1da0/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>