idmap_cache: Only touch "sid" on success in find_xid_to_sid

idmap_cache: Only touch "sid" on success in find_xid_to_sid

Why? This makes the negative mapping condition (is_null_sid) more
explicit in the code.

The callers in lookup_sid initialized "psid" anyway before, and the ones
in wb_xids2sids now do as well. This is more in line with other APIs we
have: Only touch output parameters if you have something to say.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Christof Schmitt <cs@samba.org>
Bug: https://bugzilla.samba.org/show_bug.cgi?id=13813
(cherry picked from commit 4faf3e9f6da7515fc263d79f77226d105c2f8524)