CVE-2020-25717 wb_sids2xids: move more checks to wb_sids2xids_next_sids2unix()

CVE-2020-25717 wb_sids2xids: move more checks to wb_sids2xids_next_sids2unix()

For the first run this is a no-op, but it simplified the caller.

We'll call wb_sids2xids_next_sids2unix() in a few more places in future
and it's easier to have this all within wb_sids2xids_next_sids2unix().

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14539

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14556

(cherry picked from commit 231c8d04b19a1c17937f988d142ca5c0f889d4e0)