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 <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19158)

(cherry picked from commit 3cca05cc194c0528865deea57d9e60ca3fb0e5d3)