Fix end-point shared secret for DTLS/SCTP

When computing the end-point shared secret, don't take the
terminating NULL character into account.
Please note that this fix breaks interoperability with older
versions of OpenSSL, which are not fixed.

Fixes #7956

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7957)