s3:params:lp_do_section - protect against NULL deref

s3:params:lp_do_section - protect against NULL deref

iServiceIndex may indicate an empty slot in the ServicePtrs
array. In this case, lpcfg_serivce_ok(ServicePtrs[iServiceIndex])
may trigger a NULL deref and crash. Skipping the check
here will cause a scan of the array in add_a_service() and the
NULL slot will be used safely.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15267

Signed-off-by: Andrew Walker <awalker@ixsystems.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Dec 20 18:49:54 UTC 2022 on sn-devel-184