Fix some memory bugs for implicit compressible flow solver with ldgns

Issue/feature addressed

Using LDGNS with the implicit compressible flow solver results in a segmentation fault. This MR fixes the segmentation fault, however the accuracy of the results is not guaranteed.

If LDGNS has never been properly implemented for implicit compressible navier-stokes, then code should be removed a proper error check should be instead added to the code.

Proposed solution

Implementation

Tests

Suggested reviewers

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Notes

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Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).