Fix EVP_Digest{Sign,Verify}Final() and EVP_Digest{Sign,Verify}() for provider only...

Fix EVP_Digest{Sign,Verify}Final() and EVP_Digest{Sign,Verify}() for provider only keys

For provider only keys where the initialization didn't catch, we may
end up crashing because the legacy code path didn't check that it had
support carefully enough.  This only happens if the caller didn't
check if initialization worked or not.

For the one-shot case, it's very simply handling the case where the
key has no legacy implementation an fall back to the standard
init+update+final mechanism.

While at it, EVP_DigestSignFinal() and EVP_DigestVerifyFinal() got a
slight code cleanup.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10806)