JTC trajectory end time validation fix (backport #1090)

[mergify]

The time_from_start member for a JointTrajectoryPoint is monotonically increasing, not a delta time, so I think we can just use the last one in the vector; summing them up seems incorrect. I also moved the points empty check earlier because .back().time_from_start would seg fault with an empty points vector.

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This is an automatic backport of pull request #1090 done by Mergify.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.78%. Comparing base (12a2ef2) to head (fe85da4).

Additional details and impacted files

@@            Coverage Diff             @@
##             iron    #1141      +/-   ##
==========================================
+ Coverage   86.76%   86.78%   +0.01%     
==========================================
  Files          86       86              
  Lines        7397     7399       +2     
  Branches      614      612       -2     
==========================================
+ Hits         6418     6421       +3     
  Misses        749      749              
+ Partials      230      229       -1     

Flag	Coverage Δ
unittests	86.78% <100.00%> (+0.01%)	⬆️

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Files	Coverage Δ
...ory_controller/src/joint_trajectory_controller.cpp	83.48% <100.00%> (-0.05%)	⬇️
...ory_controller/test/test_trajectory_controller.cpp	99.74% <100.00%> (+<0.01%)	⬆️

... and 3 files with indirect coverage changes