s3:winbind:idmap_ldap: fix a crash bug in idmap_ldap_unixids_to_sids (#6387)

This fixes a crash bug hit when multiple mappings were found by
the ldap search. This crash was caused by an ldap asssertion
in ldap_next_entry because was set to NULL in each iteration.

The corresponding fix was applied to the idmap_ldap_sids_to_unixids()
by Jerry in 2007 (b066668b74768d9ed547f16bf7b6ba6aea5df20a).

This fixes the crash part of bug #6387.

There is a logic part, too:
The problem currently only occurs when multiple mappings are found
for one given unixid. Now winbindd does not crash any more but
it does not correctly handle this situation. It just returns the
last mapping from the ldap search results.
This needs fixing.

Michael