Fix memory leaks in mesh loading

[jslee02]

This PR fixes the memory leaks reported in #1063.

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[codecov]

Codecov Report

Merging #1066 into release-6.5 will decrease coverage by 0.08%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release-6.5    #1066      +/-   ##
===============================================
- Coverage        56.61%   56.52%   -0.09%     
===============================================
  Files              316      316              
  Lines            24393    24396       +3     
===============================================
- Hits             13809    13790      -19     
- Misses           10584    10606      +22

Impacted Files	Coverage Δ
dart/dynamics/MeshShape.cpp	48.12% <100%> (-13.27%)	⬇️
dart/dynamics/AssimpInputResourceAdaptor.cpp	56.32% <100%> (+0.5%)	⬆️