libxml2 - security update

It was discovered that the update released for libxml2 in DSA 2978 fixing
CVE-2014-0191 was incomplete. This caused libxml2 to still fetch external
entities regardless of whether entity substitution or validation is
enabled.

In addition, this update addresses a regression introduced in DSA 3057 by
the patch fixing CVE-2014-3660. This caused libxml2 to not parse an
entity when it’s used first in another entity referenced from an
attribute value.

For Debian 6 Squeeze, these issues have been fixed in libxml2 version 2.7.8.dfsg-2+squeeze11