Fix memory corruption due to faulty refcount tracking

[azeey]

🦟 Bug fix

Summary

The memory corruption seems to occur only on DART 6.9.2 (version that comes with Ubuntu 20.04), but not 6.12 (Ubuntu 22.04), so I'm led to believe it's a bug in dart that has since been fixed. But the patch in this PR seems to avoid the problem by not storing the freejoint created when creating links. My suspicion is that this freejoint is not properly taken into account when removing skeletons.

This is blocking gazebosim/gz-sim#1836 as the test INTEGRATION_recreate_entities is reliably failing on CI. Running the test under valgrind shows that a memory corruption occurs while trying to decrement a reference count of a skeleton. With this patch, the valgrind error goes away.

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

Merging #480 (8feceb1) into ign-physics5 (b18faf2) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 8feceb1 differs from pull request most recent head 7121b72. Consider uploading reports for the commit 7121b72 to get more accurate results

@@               Coverage Diff                @@
##           ign-physics5     #480      +/-   ##
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- Coverage         76.35%   76.35%   -0.01%     
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  Files               129      129              
  Lines              5811     5810       -1     
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- Hits               4437     4436       -1     
  Misses             1374     1374              

Impacted Files	Coverage Δ
dartsim/src/SDFFeatures.cc	62.65% <ø> (-0.08%)	⬇️

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