Corrected nanostack coverity warnings

[mikaleppanen]

Summary of changes

Corrected nanostack coverity warnings.

This is mbed-os master version of:
#14146

Impact of changes

None

Migration actions required

None

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

@mikter @artokin

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[ciarmcom]

@mikaleppanen, thank you for your changes.
@artokin @mikter @ARMmbed/mbed-os-mesh @ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

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jenkins-ci/mbed-os-ci_build-cloud-example-GCC_ARM	✔️
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jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
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jobs	Status
jenkins-ci/mbed-os-ci_unittests	✔️
jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_build-greentea-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_cmake-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_cmake-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-cloud-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_build-greentea-ARM	✔️
jenkins-ci/mbed-os-ci_build-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-cloud-example-ARM	✔️
jenkins-ci/mbed-os-ci_greentea-test	✔️
jenkins-ci/mbed-os-ci_cmake-example-test	✔️