CVE-2022-2031 s4:kpasswd: Correctly generate error strings

CVE-2022-2031 s4:kpasswd: Correctly generate error strings

The error_data we create already has an explicit length, and should not
be zero-terminated, so we omit the trailing null byte. Previously,
Heimdal builds would leave a superfluous trailing null byte on error
strings, while MIT builds would omit the final character.

The two bytes added to the string's length are for the prepended error
code.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15047
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15049
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15074

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
[jsutton@samba.org Removed MIT KDC 1.20-specific knownfails]