If you call X509_add_cert with cert == NULL and the X509_ADD_FLAG_UP_REF

If you call X509_add_cert with cert == NULL and the X509_ADD_FLAG_UP_REF
flag, it will сrash to X509_up_ref.  Passing NULL here is not valid,
return 0 if cert == NULL.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/26267)