Fix memory leak in generator plugin code

[mjcarroll]

🦟 Bug fix

Summary

Use a managed pointer rather than a raw pointer to ensure that cleanup happens correctly.

Discovered with asan.

[codecov[bot]]

Codecov Report

Merging #404 (89d1040) into gz-msgs10 (edd9632) will not change coverage. Report is 1 commits behind head on gz-msgs10. The diff coverage is 100.00%.

:exclamation: Current head 89d1040 differs from pull request most recent head dae88a1. Consider uploading reports for the commit dae88a1 to get more accurate results

@@            Coverage Diff             @@
##           gz-msgs10     #404   +/-   ##
==========================================
  Coverage      97.26%   97.26%           
==========================================
  Files             27       27           
  Lines           1172     1172           
==========================================
  Hits            1140     1140           
  Misses            32       32           

Files	Coverage Δ
core/generator/Generator.cc	92.64% <100.00%> (ø)