Fix IDAKLU segfault with output variables

[MarcBerliner]

Description

Fixed a memory issue with output variables reported in #4375

Fixes #4375

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• [x] Bug fix (non-breaking change which fixes an issue)

Key checklist:

• [x] No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• [x] All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• [x] The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• [ ] Code is commented, particularly in hard-to-understand areas
• [ ] Tests added that prove fix is effective or that feature works

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.46%. Comparing base (4d391fa) to head (efbed0e). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4379 +/- ## ======================================== Coverage 99.46% 99.46% ======================================== Files 289 289 Lines 22200 22200 ======================================== Hits 22081 22081 Misses 119 119 ```

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