crypto: move validation logic from cipher_get to cipher_valid

crypto: move validation logic from cipher_get to cipher_valid

With cipher validation performed in cipher_get(), a cipher is never
returned in any case if some check fails.

This prevents OpenVPN from operating on all ciphers provided by the SSL
library, like printing them to the user.

Move the validation logic to cipher_valid() so that checks are performed
only when OpenVPN really want to know if a cipher is usable or not.

Fixes: ce2954a0 ("Remove cipher_kt_t and change type to const char* in
API")
Cc: Arne Schwabe <arne@rfc2549.org>
Cc: David Sommerseth <davids@openvpn.net>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20220203193655.28791-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23713.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>