util: Fix off-by-one error in message about overflow

util: Fix off-by-one error in message about overflow

len includes space for the NUL character, so the calculation needs to
take the NUL character into account.

While touching this, drop unnecessary casts by updating format string
and update to modern debug macro.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Fri Jul  5 02:24:52 UTC 2019 on sn-devel-184