Terminate decode at end of string and fix mem leak

Seems that we have to NULL terminate the buffer for
xmlStringLenDecodeEntitites() as otherwise we might end up having extra
data at the end of returned buffer.  (Somehow the length parameter is
not respected always, even if it is the proper size returned by the
zip_fread() - header_skip).

Also free the buffer returned by xmlStringLenDecodeEntitites().

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>