Fix bullet classic entity management

[mxgrey]

This PR fixes the entity management features for the bullet classic plugin. It also fixes the seg fault that was reported in this other PR for gz-sim.

[codecov]

Codecov Report

Merging #393 (91ec7ef) into main (f226dc7) will decrease coverage by 0.21%.
The diff coverage is 14.28%.

❗ Current head 91ec7ef differs from pull request most recent head db25242. Consider uploading reports for the commit db25242 to get more accurate results

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
- Coverage   76.77%   76.56%   -0.22%     
==========================================
  Files         128      128              
  Lines        5628     5657      +29     
==========================================
+ Hits         4321     4331      +10     
- Misses       1307     1326      +19     

Impacted Files	Coverage Δ
bullet/src/EntityManagementFeatures.cc	9.39% <0.72%> (-3.20%)	⬇️
bullet/src/Base.hh	74.57% <95.45%> (+26.04%)	⬆️
bullet/src/SDFFeatures.cc	45.06% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f226dc7...db25242. Read the comment docs.