Fix possible NAN on some extreme tracks

src/physics/btKart.cpp

@@ -981,6 +981,8 @@ void btKart::adjustSpeed(btScalar min_speed, btScalar max_speed)
             btVector3 upright_component = normal * normal.dot(velocity);
             // Subtract the upright velocity component,
             btVector3 v = velocity - upright_component;
+            if (!v.fuzzyZero())
+            {
                 const float velocity_ratio = min_speed / v.length();
                 // Scale the velocity in the plane, then add the upright component
                 // of the velocity back in.
@@ -988,6 +990,7 @@ void btKart::adjustSpeed(btScalar min_speed, btScalar max_speed)
                                                 + upright_component );
             }
         }
+    }
     else if (speed >0 && max_speed >= 0 && speed > max_speed)
     {
         const float velocity_ratio = max_speed / speed;