We change the load_key() and load_pubkey() functions to make them more
consistent with the load_keyparams() function modified as a result of
PR #13317.
The error message on a NULL key is removed, because an error message has
already been displayed by load_key_certs_crls().
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/13355)