chore: fix C lint errors (issue #6887)

[LalitNarayanYadav]

This PR resolves the C lint warning reported in issue #6887.

Changes:

• Replaced use of uninitialized VLA x in benchmark.length.c with dynamically allocated memory.
• Ensured proper initialization and deallocation of x to comply with C standards and avoid undefined behavior.

Related Issues:

Resolves #6887

Checklist

• Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dmeanvarpn	$\color{green}518/518$ $\color{green}+100.00\%$	$\color{green}45/45$ $\color{green}+100.00\%$	$\color{green}4/4$ $\color{green}+100.00\%$	$\color{green}518/518$ $\color{green}+100.00\%$

The above coverage report was generated for the changes in this PR.

[kgryte]

Ref: #6895

[stdlib-bot]

Thank you for working on this pull request. However, we cannot accept your contribution as the issue this pull request seeks to resolve has already been addressed in a different pull request or commit.

Thank you again for your interest in stdlib, and we look forward to reviewing your future contributions.