Prevent overflow when calculating ulog block size

Prevent overflow when calculating ulog block size

In kdb_log.c:resize(), log an error and fail if the update size is
larger than the largest possible block size (2^16-1).

CVE-2025-24528:

In MIT krb5 release 1.7 and later with incremental propagation
enabled, an authenticated attacker can cause kadmind to write beyond
the end of the mapped region for the iprop log file, likely causing a
process crash.

[ghudson@mit.edu: edited commit message and added CVE description]

ticket: 9159 (new)
tags: pullup
target_version: 1.21-next