Fix message for too long tls-crypt-v2 metadata

Fix message for too long tls-crypt-v2 metadata

The current code only checks if the base64-encoded metadata is at most
980 characters. However, that can encode up to 735 bytes of data, while
only up to 733 bytes are allowed. When passing 734 or 735 bytes, openvpn
prints a misleading error message saying that the base64 cannot be
decoded.

This patch checks the decoded length to show an accurate error message.

v2: Remove now-unused macro and fix an off-by-one error.

Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20221214153414.12671-1-maximilian.fillinger@foxcrypto.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25694.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>