Fix bounds check in read_key()

Fix bounds check in read_key()

The bounds check in read_key() was performed after using the value, instead
of before.  If 'key-method 1' is used, this allowed an attacker to send a
malformed packet to trigger a stack buffer overflow.

Fix this by moving the input validation to before the writes.

Note that 'key-method 1' has been replaced by 'key method 2' as the default
in OpenVPN 2.0 (released on 2005-04-17), and explicitly deprecated in 2.4
and marked for removal in 2.5.  This should limit the amount of users
impacted by this issue.

CVE: 2017-12166
Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <80690690-67ac-3320-1891-9fecedc6a1fa@fox-it.com>
URL: https://www.mail-archive.com/search?l=mid&q=80690690-67ac-3320-1891-9fecedc6a1fa@fox-it.com
Signed-off-by: David Sommerseth <davids@openvpn.net>
(backport of git master commit 3b1a61e9fb27213c46f76312f4065816bee8ed01)