TractionLimiter: Fix wrong input checks

[christophfroehlich]

While working on ros-controls/control_toolbox#212 and writing tests, I found some issues with the TractionLimiter of the tricycle_controller.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.47%. Comparing base (f519170) to head (6c7bd61).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage   80.89%   81.47%   +0.57%     
==========================================
  Files         109      110       +1     
  Lines        9696     9821     +125     
  Branches      842      858      +16     
==========================================
+ Hits         7844     8002     +158     
+ Misses       1574     1541      -33     
  Partials      278      278              

Flag	Coverage Δ
unittests	81.47% <100.00%> (+0.57%)	⬆️

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

Files with missing lines	Coverage Δ
tricycle_controller/src/traction_limiter.cpp	95.71% <100.00%> (+69.71%)	⬆️
tricycle_controller/test/test_traction_limiter.cpp	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes