libxml2: fix CVE-2015-7942 and CVE-2015-8035

CVE-2015-7942 libxml2: heap-based buffer overflow in xmlParseConditionalSections()
CVE-2015-8035 libxml2: DoS when parsing specially crafted XML document if XZ support is enabled

[YOCTO #8641]

(From OE-Core master rev: 27de51f4)

Signed-off-by: Armin Kuster <akuster@mvista.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Joshua Lock <joshua.lock@collabora.co.uk>