Fix memory leak in DNS cache

[boraozgen]

Summary of changes

Fixes the memory leak that happens when DNS cache is expired (#13398). The address member of dns_cache is forgotten to be freed before freeing the struct.

I did not run any tests on this change, but it fixed the memory leak I was observing using the mbed_stats_heap_get() API. I assume that the scenario of waiting DNS cache expiry is not covered in the tests at all. I checked "Covered by existing mbed-os tests" below but maybe new tests are required to test this scenario.

Impact of changes

Migration actions required

Documentation

No update required.

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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]

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

[0xc0170]

CI started

[mbed-ci]

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jobs	Status
jenkins-ci/mbed-os-ci_unittests	✔️
jenkins-ci/mbed-os-ci_build-ARM	✔️
jenkins-ci/mbed-os-ci_build-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_dynamic-memory-usage	✔️
jenkins-ci/mbed-os-ci_cloud-client-pytest	✔️
jenkins-ci/mbed-os-ci_greentea-test	✔️