LZW decompress: fix for CVE-2011-2895

Specially crafted LZW stream can crash an application using libXfont
that is used to open untrusted font files.  With X server, this may
allow privilege escalation when exploited

Reviewed-by: Matthieu Herrb <matthieu.herrb@laas.fr>
Signed-off-by: Matthieu Herrb <matthieu.herrb@laas.fr>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>