Fix lock_guard scope in DataWriterImpl::get_listener_for

[MiguelCompany]

Description

During the review of a backport of #4277, I discovered that this improvement could be made.

Opened the PR so I don't forget about the changes. Leaving as draft and with skip-ci till I create a task for it.

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• 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
• Any new/modified methods have been properly documented using Doxygen.
• Changes are ABI compatible.
• Changes are API compatible.
• New feature has been added to the versions.md file (if applicable).
• New feature has been documented/Current behavior is correctly described in the documentation.
• 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.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.