Trust the returned value from EVP_PKEY_get_default_digest_name()!  It
mimics exactly the values that EVP_PKEY_get_default_digest_nid() is
supposed to return, and that value should simply be passed unchanged.
Callers depend on it.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12586)