Do not error when md_kt_size() is called with mdname="none"

Do not error when md_kt_size() is called with mdname="none"

An easy way to trigger this error is to run an otherwise working setup
(at say verb = 4) with increased verbosity of verb >= 7 and using a GCM
cipher (e.g., AES-256-GCM). It will cause a fatal exit while printing the
cipher and hmac in key2_print().

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220121185752.14138-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23631.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>