cleanse stack variable in blake2[b|s] finalization

cleanse stack variable in blake2[b|s] finalization

If the output of a blake2[b|s] digest isn't a multipl of 8, then a stack
buffer is used to compute the final output, which is left un-zeroed
prior to return, allowing the potential leak of key data.  Ensure that,
if the stack variable is used, it gets cleared prior to return.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23173)

(cherry picked from commit 8b9cf1bc2c3085b6e9493a057209ffd0bddf48a6)