Fix atomic variables in JTC (backport #1749)

[mergify]

Explicitly initialize them, should fix the error reported with #1720 (comment)

I don't know why the tests did not cover this?

Furthermore, I saw that subscriber_is_active_ should be atomic too, as it is used in both threads.

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This is an automatic backport of pull request #1749 done by Mergify.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.08%. Comparing base (4b488cb) to head (81c3b1a).
Report is 1 commits behind head on humble.

Additional details and impacted files

@@           Coverage Diff           @@
##           humble    #1765   +/-   ##
=======================================
  Coverage   67.08%   67.08%           
=======================================
  Files         122      122           
  Lines       14506    14506           
  Branches     8964     8954   -10     
=======================================
  Hits         9732     9732           
  Misses       1461     1461           
  Partials     3313     3313           

Flag	Coverage Δ
unittests	67.08% <ø> (ø)

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Files with missing lines	Coverage Δ
...jectory_controller/joint_trajectory_controller.hpp	100.00% <ø> (ø)

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