Fix SSL_export_keying_material() for DTLS1_BAD_VER

Commit d8e8590e

 ("Fix missing return value checks in SCTP") made the
DTLS handshake fail, even for non-SCTP connections, if
SSL_export_keying_material() fails. Which it does, for DTLS1_BAD_VER.

Apply the trivial fix to make it succeed, since there's no real reason
why it shouldn't even though we never need it.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>