idmap: centrally check that unix IDs returned by the idmap backends are in range

idmap: centrally check that unix IDs returned by the idmap backends are in range

Note: in the long run, it might be good to move this kind of
exit check (before handing the result back to the client)
to the parent winbindd code.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12155

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Michael Adam <obnox@samba.org>
Autobuild-Date(master): Wed Aug 17 01:21:39 CEST 2016 on sn-devel-144