Minor optimisation to ATCmdParser unit test code

[DavidLin1577]

Summary of changes

This is a minor optimisation to the unit test code:

1. The first buf initialisation was missing (although not required by the test code)
2. The second buf is zero initialised instead of using memset. The code is more readable and portable.

Impact of changes

Migration actions required

Documentation

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Pull request type

[*] 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)
[*] 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]

@DavidLin1577, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[evedon]

Thank you for your change.

The first buf initialisation is not required but does not harm.
The second initialisation is required by the test. Although the memset implementation was fine, it is better to zero initialise the array. (There was a recent change to increase buf size and memset had to be modified as well).

Could you update :

• the PR description to explain what you changed and why, see guidelines here https://os.mbed.com/docs/mbed-os/v5.15/contributing/workflow.html#guidelines-for-github-pull-requests
• the Test Results section to indicate that test is Covered by existing mbed-os test.

[DavidLin1577]

Thanks so much for evedon:
As you advise, I had readed the guidelines, and updated the comment, thank you.

[evedon]

@DavidLin1577 Re-worded the title and PR description for readability. PR approved.

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

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

[mergify]

This PR does not contain release version label after merging.

[0xc0170]

It looks like we shall first mark the version the merge. Timing here might be involved. As I tested this on another PR that I marked version first then merged, no release review required. This was done other way around. Anyway, will report this to mergify