Fix a crash due to an invalid pointer

[nkoenig]

🦟 Bug fix

Summary

A crash could occur if _parent is null and the joint type is FreeJoint.

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 #486 (82b43a0) into ign-physics2 (0ac0be1) will increase coverage by 0.02%.
The diff coverage is 96.15%.

❗ Current head 82b43a0 differs from pull request most recent head d7d0ea7. Consider uploading reports for the commit d7d0ea7 to get more accurate results

@@               Coverage Diff                @@
##           ign-physics2     #486      +/-   ##
================================================
+ Coverage         83.15%   83.18%   +0.02%     
================================================
  Files               108      108              
  Lines              4210     4210              
================================================
+ Hits               3501     3502       +1     
+ Misses              709      708       -1     

Impacted Files	Coverage Δ
dartsim/src/Base.hh	99.01% <ø> (ø)
dartsim/src/KinematicsFeatures.cc	72.00% <ø> (ø)
dartsim/src/ShapeFeatures.cc	43.04% <ø> (ø)
dartsim/src/SimulationFeatures.cc	82.90% <ø> (ø)
dartsim/src/SimulationFeatures.hh	100.00% <ø> (ø)
dartsim/src/plugin.cc	100.00% <ø> (ø)
include/gz/physics/detail/BoxShape.hh	100.00% <ø> (ø)
include/gz/physics/detail/CanReadData.hh	100.00% <ø> (ø)
include/gz/physics/detail/CanWriteData.hh	100.00% <ø> (ø)
include/gz/physics/detail/Cloneable.hh	100.00% <ø> (ø)
... and 92 more

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