DTLSSocket - destruction while handshaking lead to error

[LukasKarel]

Summary of changes

When a DTLSSocket is deleted (destructor was called) while the handshake procedure is ongoing, a event is triggered after the timeout. This should not happen as the object is not valid anymore.

Impact of changes

Migration actions required

Documentation

None.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@LukasKarel, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Add please information to the commit message (what is the issue about and how it fixes it).

Does this need also unittest addition?

Pull request has been modified.

[LukasKarel]

Add please information to the commit message (what is the issue about and how it fixes it).

Does this need also unittest addition?

I am not sure how this should be tested correctly, because a server is needed which does not respond to a DTLS handshake request.

[0xc0170]

Looks fine to me but I would like to have connectivity team reviewed

[mbed-ci]

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