s390x: fix ecx derive
s390x: fix ecx derive

Derivation via ecx-methods did not properly set the length.  Make the code
more similar to common code and set the length property.

Signed-off-by: Juergen Christ <jchrist@linux.ibm.com>

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
Reviewed-by: Hugo Landau <ce9fa1e55a72fc9b85dd06e47ea2c07848fb7d2c@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19158)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl