Fix RobotTrajectory's copy constructor

[rhaschke]

Fixup for 01ff035 (#1760)
Fixes #1833.

[codecov-io]

Codecov Report

Merging #1834 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1834   +/-   ##
======================================
  Coverage    48.5%   48.5%           
======================================
  Files         298     298           
  Lines       23302   23302           
======================================
  Hits        11302   11302           
  Misses      12000   12000

Impacted Files	Coverage Δ
...eit_core/robot_trajectory/src/robot_trajectory.cpp	41.81% <ø> (ø)	⬆️

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