samba-o3: fix -Werror=strict-overflow error in lib/ldb-samba/ldb_ildap module

samba-o3: fix -Werror=strict-overflow error in lib/ldb-samba/ldb_ildap module

samba-o3 test failed in ubuntu:16.04 docker container:

    ==> /home/samba/samba/samba-o3.stderr <==
    ../../lib/ldb-samba/ldb_ildap.c: In function ‘ildb_handle_request’:
    ../../lib/ldb-samba/ldb_ildap.c:535:2: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
      for (i = 0; i < n; i++) {
      ^
    ../../lib/ldb-samba/ldb_ildap.c:579:2: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
      for (i = 0; i < n; i++) {
      ^
    cc1: all warnings being treated as errors

Change type to mute errors.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>