Check that ball speed is positive

Signed-off-by: Stephan Pleines <pleines.stephan@gmail.com>
Change-Id: I032e427e6dc0395b0664eb69d2eebbd245699026
diff --git a/frc971/control_loops/aiming/aiming.cc b/frc971/control_loops/aiming/aiming.cc
index 4cf9255..572ed1c 100644
--- a/frc971/control_loops/aiming/aiming.cc
+++ b/frc971/control_loops/aiming/aiming.cc
@@ -54,6 +54,8 @@
 }  // namespace
 
 TurretGoal AimerGoal(const ShotConfig &config, const RobotState &state) {
+  CHECK(config.ball_speed_over_ground > 0.0)
+      << ": Ball speed must be positive.";
   TurretGoal result;
   // This code manages compensating the goal turret heading for the robot's
   // current velocity, to allow for shooting on-the-fly.