mbed-coap buffer overflow fix

[ristohuhtala]

Summary of changes

Fixed mbed-coap buffer overflow when paring CoAP message. Changed buffer handling when CoAP message is parsed. Buffer size is checked to avoid overflow.

Impact of changes

Migration actions required

Documentation

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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

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

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Reviewers

@anttiylitokola @yogpan01

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[ciarmcom]

@ristohuhtala, thank you for your changes.
@anttiylitokola @yogpan01 @ARMmbed/mbed-os-maintainers please review.

[ristohuhtala]

Added CHANGELOG.md changes to this PR like @yogpan01 suggested.

[0xc0170]

As #12147 contains as well the changelog, please remove it from this one. This will be merged first, and 12147 right after containing the changelog.

[ristohuhtala]

As #12147 contains as well the changelog, please remove it from this one. This will be merged first, and 12147 right after containing the changelog.

I will remove it

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 11 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_cloud-client-pytest

[0xc0170]

mbed-os-ci_cloud-client-pytest restarted

[bulislaw]

I'm assuming @JanneKiiskila will want it in next patch?