Fix bug where address of local variable was returned

[azeey]

🦟 Bug fix

Summary

This patch returns a malloc'd copy instead.

This was introduced in #218. homebrew had a warning https://build.osrfoundation.org/job/gz_launch-ci-pr_any-homebrew-amd64/24/clang/, but our Linux CI was green: https://build.osrfoundation.org/job/gz_launch-ci-pr_any-focal-amd64/16/

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 56.22%. Comparing base (17835ca) to head (176b9f6).

Files	Patch %	Lines
src/cmd/gz.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           gz-launch6     #252   +/-   ##
===========================================
  Coverage       56.22%   56.22%           
===========================================
  Files               4        4           
  Lines             418      418           
===========================================
  Hits              235      235           
  Misses            183      183           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.