Don't use msg_ field of realtime publisher

[christophfroehlich]

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.13%. Comparing base (4b1f45e) to head (32a3885).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1947      +/-   ##
==========================================
- Coverage   85.15%   85.13%   -0.03%     
==========================================
  Files         143      143              
  Lines       13763    13742      -21     
  Branches     1200     1200              
==========================================
- Hits        11720    11699      -21     
  Misses       1636     1636              
  Partials      407      407              

Flag	Coverage Δ
unittests	85.13% <100.00%> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
...iff_drive_controller/src/diff_drive_controller.cpp	82.67% <100.00%> (-0.06%)	⬇️
...ive_controller/test/test_diff_drive_controller.cpp	93.97% <ø> (-0.35%)	⬇️
...roadcaster/src/force_torque_sensor_broadcaster.cpp	89.68% <100.00%> (ø)
...ntroller/test/test_trajectory_controller_utils.hpp	84.61% <ø> (ø)
...nclude/tricycle_controller/tricycle_controller.hpp	100.00% <ø> (ø)

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