Fixed crashing nsapi_dns unittest

[AnttiKauppila]

Added missing "return" for NetworkStackMock's get_stack() method

Description

Summary of change

nsapi_dns unittest is crashing in local environment due to missing return in NetworkStackMock's get_stack() function. This PR adds the missing return and fixes the problem.

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

[] 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

@michalpasztamobica

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Release Notes

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 1 of 1 test jobs passed
Build number : 1
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