An incorrect error code addition was spotted by the new invasive testing
infrastructure whereby pk_get_pk_alg will always return a high level
error or zero and pk_parse_key_pkcs8_unencrypted_der will try to add
another high level error, resulting in a garbage error code.
Apply the same fix from ae3741e8a to fix the bug.
Signed-off-by: Chris Jones <christopher.jones@arm.com>
