Fix NULL pointer access caused by X509_ATTRIBUTE_create()

Fix NULL pointer access caused by X509_ATTRIBUTE_create()

When X509_ATTRIBUTE_create() receives an invalid NID (e.g., -1), return
failure rather than silently constructing a broken X509_ATTRIBUTE object
that might cause NULL pointer accesses later on.  This matters because
X509_ATTRIBUTE_create() is used by API functions like PKCS7_add_attribute(3)
and the NID comes straight from the user.

This bug was found while working on LibreSSL documentation.

Reviewed-by: Theo Buehler <tb@openbsd.org>
CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/12052)