sss_override: do not free ldb_dn in get_object_dn()
When only str_dn is requested, ldb_dn is freed. This triggers access
after free since str_dn is part of ldb_dn talloc context.
Reviewed-by: Pavel Reichl <preichl@redhat.com>
(cherry picked from commit 2341c8ccfe6225ee4ac5904c177a9200ba617a04)