Fix pdb_samba_dsdb build on Ubuntu 16.04 with -O3 -Werror=strict-overflow

Fix pdb_samba_dsdb build on Ubuntu 16.04 with -O3 -Werror=strict-overflow

../source3/passdb/pdb_samba_dsdb.c: In function ‘pdb_samba_dsdb_set_trusteddom_pw’:
../source3/passdb/pdb_samba_dsdb.c:2778:6: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
   if (i == 0) {
      ^
cc1: all warnings being treated as errors

gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.10)

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Sep 20 01:29:48 CEST 2018 on sn-devel-144