Anon Log feature broken

#### Issue details
The log anonymizer feature has been broken since at least the transition to ArduPilot 4.2 (and possibly at some point during 4.1 development). The file is created, but it is unreadable by log analysis tools (screenshot attached with Mission Planner KeyNotFound exception text).

My last successful use of the feature was Sep 2021 with a log from Rover 4.1.0-RC1.


I used to use the feature extensively for sharing with the community and have been remiss in opening the issue here - it has been present for quite some time.

#### Version
1.3.77 build 1.3.8130.42812

#### Platform
[x ] All

#### Airframe type
N/A

#### Hardware type
N/A

#### Logs
Any .bin log from a version newer than ArduPilot 4.1 should replicate results.

Reproduce the issue with any .bin log from an ArduPilot version newer than 4.1 and compare the anonymized output with a log-analysis tool. Trace the log anonymizer entry point and identify why the generated file is unreadable, using the reported Mission Planner KeyNotFound exception as the failure signal. Done means anonymized logs can again be opened by the affected analysis tools.

Written by the indexing model from the issue text.