[AtModem] Fix NullReferenceException in AtChannel.ProcessMessage

[rikkreeftenberg]

Description

The _currentCommand and the _currentResponse are used troughout the AtChannel class. They are set to null in the 'finally' of the AtChannel.SendFullCommand. But in very few occasions (once every hour or so) the _currentCommand is used by another thread after it has been set to null, resulting in a NullReferenceException. For example in HandleFinalResponse. This is the same multi-threading problem like the one fixed in PR #1216.

We can fix this by placing a lock around the call to AtChannel.ProcessMessage so access to _currentCommand and _currentresponse is mutually exclusive to setting them to null. So NullReference cannot occur.

This also discussed with @Ellerbach in https://discord.com/channels/478725473862549535/1320052446227071116/1323303047895449642

Signed-off-by: Rik Kreeftenberg rik.kreeftenberg@spinsoft.nl

Motivation and Context

This fixes the last NullreferenceException that I came accross during my testing of the AtModem library.

• Addresses AtModem Http connection is not stable when POST-ing large numbers of messages over a long time Home#1579

How Has This Been Tested?

I have tested this by POST'ing thousands of messages to an Azure Function app for a day and checking if there are any uncaught exceptions written to a logfile created by connecting Putty to the debug port. There weren't.. ;-)

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• My code follows the code style of this project (only if there are changes in source code).
• [] My changes require an update to the documentation (there are changes that require the docs website to be updated).
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• I have read the CONTRIBUTING document.
• I have tested everything locally and all new and existing tests passed (only if there are changes in source code).
• [] I have added new tests to cover my changes.

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