Don't use msg_ field of realtime publisher (backport #1947)

[mergify]

Some things I haven't seen in #1830, see ros-controls/realtime_tools#371

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This is an automatic backport of pull request #1947 done by Mergify.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.66%. Comparing base (b1f7bbe) to head (6d9b03b).
⚠️ Report is 1 commits behind head on jazzy.

Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy    #1948      +/-   ##
==========================================
- Coverage   84.68%   84.66%   -0.03%     
==========================================
  Files         138      138              
  Lines       13429    13408      -21     
  Branches     1197     1197              
==========================================
- Hits        11373    11352      -21     
  Misses       1643     1643              
  Partials      413      413              

Flag	Coverage Δ
unittests	84.66% <100.00%> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
...iff_drive_controller/src/diff_drive_controller.cpp	75.97% <100.00%> (-0.08%)	⬇️
...ive_controller/test/test_diff_drive_controller.cpp	93.97% <ø> (-0.35%)	⬇️
...roadcaster/src/force_torque_sensor_broadcaster.cpp	89.76% <100.00%> (ø)
...ntroller/test/test_trajectory_controller_utils.hpp	84.19% <ø> (ø)
...nclude/tricycle_controller/tricycle_controller.hpp	100.00% <ø> (ø)

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