Fix coverity alert on use of uninitialised data

The function `ossl_blake2b_param_init` should initialise only, and not
read the data it is initialising

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22282)