(ITS#5959) back-meta doesn't detect illegal values

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.4.14/HEAD
OS: Linux
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (76.91.220.157)
Submitted by: hyc


back-meta doesn't check the return code from the normalizer when it's
normalizing the values of an attribute. As such, it can leave NULL slots in
a->a_nvals even though a->a_vals slots are all non-NULL.

I was going to try just dup'ing the a->a_vals value into the slot, but I see
that in back-ldap for the same situation, the entire attribute is just dropped.
I suppose that's the safer path.