apparmor: check that xindex is in trans_table bounds

commit 23ca7b640b4a55f8747301b6bd984dd05545f6a7 upstream.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Seth Arnold <seth.arnold@canonical.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Willy Tarreau <w@1wt.eu>