[jog_arm] fix access past end of array bug

[tylerjw]

Description

Manually tracing the code I'm not sure how this is happening but it is and this causes odd behavior (sending trajectory data from seemingly random data).

[AndyZe]

IMO there's no reason not to add a check like this 👍

[codecov]

Codecov Report

Merging #2155 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2155      +/-   ##
==========================================
- Coverage   57.94%   57.90%   -0.04%     
==========================================
  Files         327      327              
  Lines       25640    25640              
==========================================
- Hits        14857    14848       -9     
- Misses      10783    10792       +9     

Impacted Files	Coverage Δ
...veit_experimental/moveit_jog_arm/src/jog_calcs.cpp	71.10% <0.00%> (-0.26%)	⬇️
.../ompl_interface/src/detail/constrained_sampler.cpp	46.15% <0.00%> (-17.95%)	⬇️
...ove_group_interface/src/wrap_python_move_group.cpp	39.87% <0.00%> (-3.17%)	⬇️
...anning_scene_monitor/src/current_state_monitor.cpp	51.28% <0.00%> (-1.54%)	⬇️
.../move_group_interface/src/move_group_interface.cpp	47.86% <0.00%> (-0.22%)	⬇️
...meterization/work_space/pose_model_state_space.cpp	82.99% <0.00%> (+0.68%)	⬆️
...ipulation/pick_place/src/manipulation_pipeline.cpp	77.35% <0.00%> (+0.94%)	⬆️
moveit_core/robot_model/src/joint_model_group.cpp	61.81% <0.00%> (+2.42%)	⬆️
...e/src/parameterization/model_based_state_space.cpp	73.10% <0.00%> (+2.75%)	⬆️

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