Fix a bug in spline drivetrain.

Only allow a spline to be stopped if it has actually started.
Until now, a spline was 'started' as soon as it finished planning.

Change-Id: I083288703708f8d36c8bc7930bf465ff1e512b5c
diff --git a/frc971/control_loops/drivetrain/splinedrivetrain.h b/frc971/control_loops/drivetrain/splinedrivetrain.h
index 3e47f5a..d70cb6d 100644
--- a/frc971/control_loops/drivetrain/splinedrivetrain.h
+++ b/frc971/control_loops/drivetrain/splinedrivetrain.h
@@ -70,6 +70,7 @@
 
   int32_t current_spline_handle_ = 0;  // Current spline told to excecute.
   int32_t current_spline_idx_ = 0;     // Current executing spline.
+  bool has_started_execution_ = false;
 
   ::std::unique_ptr<DistanceSpline> current_distance_spline_;
   ::std::unique_ptr<Trajectory> current_trajectory_;