[21670] Avoid Data Race in Reader Locator (backport #5833)

[mergify]

Description

This PR modifies find_local_reader method to assign the local_reader shared pointer within the function, instead of returning the value. In this way, all operations are protected by the RTPSDomain mutex and no data race can occur.

This Data Race is reported by DDS Router CI.

Contributor Checklist

• Commit messages follow the project guidelines.

• The code follows the style guidelines of this project.

• N/A Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally

• N/A Any new/modified methods have been properly documented using Doxygen.

• N/A Any new configuration API has an equivalent XML API (with the corresponding XSD extension)

• N/A Changes are backport compatible: they do NOT break ABI nor change library core behavior.

• N/A Changes are API compatible.

• N/A New feature has been added to the versions.md file (if applicable).

• N/A New feature has been documented/Current behavior is correctly described in the documentation.

• N/A Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• If this is a critical bug fix, backports to the critical-only supported branches have been requested.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

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This is an automatic backport of pull request #5833 done by [Mergify](https://mergify.com).