Change returned -2 to 0 in EVP_Digest{Sign,Verify}Init()

Change returned -2 to 0 in EVP_Digest{Sign,Verify}Init()

The returned -2 was to mark when these operations are unsupported.
However, that breaks away from the previous API and expectations, and
there's not enough justification for that not being zero.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10815)