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.

---

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 84.93%. Comparing base (8f8880e) to head (f35fe9e).
Report is 1 commits behind head on jazzy.

Additional details and impacted files

@@           Coverage Diff           @@
##            jazzy    #1766   +/-   ##
=======================================
  Coverage   84.92%   84.93%           
=======================================
  Files         127      127           
  Lines       12175    12174    -1     
  Branches     1038     1037    -1     
=======================================
  Hits        10340    10340           
  Misses       1497     1497           
+ Partials      338      337    -1     

Flag	Coverage Δ
unittests	84.93% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.