winbindd: in wb_lookupsids return domain name if we have it

winbindd: in wb_lookupsids return domain name if we have it

When doing a SID to xid mapping for an unknown SID, the idmap child gets
passed a lsa_RefDomainList with an empty domain name (ie ""). This is
coming from LsaLookupSids() and causes the mapping request to end up in
the default idmap domain.

Example request with domain name "":

  wbint_Sids2UnixIDs: struct wbint_Sids2UnixIDs
     in: struct wbint_Sids2UnixIDs
         domains                  : *
             domains: struct lsa_RefDomainList
                 count                    : 0x00000001 (1)
                 domains                  : *
                     domains: ARRAY(1)
                         domains: struct lsa_DomainInfo
                             name: struct lsa_StringLarge
                                 length                   : 0x0000 (0)
                                 size                     : 0x0002 (2)
                                 string                   : *
                                     string                   : ''
                             sid                      : *
                                 sid                      : S-1-5-21-3152989960-574718769-2188965058
                 max_size                 : 0x00000020 (32)
         ids                      : *
             ids: struct wbint_TransIDArray
                 num_ids                  : 0x00000001 (1)
                 ids: ARRAY(1)
                     ids: struct wbint_TransID
                         type                     : ID_TYPE_NOT_SPECIFIED (0)
                         domain_index             : 0x00000000 (0)
                         rid                      : 0x000029aa (66666)
                         xid: struct unixid
                             id                       : 0xffffffff (4294967295)
                             type                     : ID_TYPE_NOT_SPECIFIED (0)

In _wbint_Sids2UnixIDs() we call idmap_find_domain_with_sid() with the
domain name "" and this triggers use of the default idmap domain which
in case of idmap_autorid will allocate an id from a idmap_autorid range.

If we know the domain, ensure we return it for SIDs were the SID was not
found but the domain of the SID was found. Callers like sids2xids depend
on the domain name and returning an empty string "" for valid domain can
trigger unwanted idmap range allocations.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11961

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>