lib/param: clang: Fix 'dereference of a null pointer' warning

lib/param: clang: Fix 'dereference of a null pointer' warning

Fixes:

lib/param/loadparm.c:3325:36: warning: Access to field 'szService' results in a dereference of a null pointer (loaded from variable 'service') <--[clang]
        return lpcfg_string((const char *)service->szService)

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Autobuild-User(master): Noel Power <npower@samba.org>
Autobuild-Date(master): Wed Jun 26 11:53:08 UTC 2019 on sn-devel-184