MQTTAsync_processCommand crashes when a previously persisted PUBLISH command cannot be retrieved

[DavidM-Fox]

Describe the bug MQTTAsync_processCommand crashes when a previously persisted PUBLISH command cannot be retrieved. In our case, the key string got corrupted so the outcome was as if the file was missing. This causes a nullptr dereference down the line within MQTTProtocol_storePublication.

To Reproduce I'm not able to reproduce the corrupted key string issue, but I can force the bug to appear by:

1. Bring the broker down
2. Persist some commands
3. Delete some of the persisted commands
4. Start the broker, on automatic reconnect the crash appears

Expected behavior MQTTAsync_processCommand should have additional handling in place for PUBLISH pget failures. I'm not sure how the other command types are handled, but I would think it should just bail out if the persistence fails to retrieve a PUBLISH command.

Screenshots

Environment

• Microsoft Visual Studio Professional 2022 (64-bit) - LTSC 17.4 (Version 17.4.7)
• Windows SDK version 10.0.22000.0 to target Windows 10.0.19045.
• MSVC_VERSION 1934 (v143 toolset)

Additional context We are using the paho mqtt cpp library. I don't have a log file to show at the moment.