fix LandmarkPolyline2D return state by copy

src/landmark_polyline_2D.cpp

@@ -60,7 +60,7 @@ void LandmarkPolyline2D::setLast(const Eigen::VectorXs& _point, bool _defined)
     	defineExtreme(true);
 }
 
-const Eigen::VectorXs& LandmarkPolyline2D::getPointVector(int _i) const
+const Eigen::VectorXs LandmarkPolyline2D::getPointVector(int _i) const
 {
 	//std::cout << "LandmarkPolyline2D::getPointVector: " << _i << std::endl;
 	//std::cout << "First: " << first_id_ << " - size: " << point_state_ptr_vector_.size() << std::endl;

src/landmark_polyline_2D.h

@@ -72,7 +72,7 @@ class LandmarkPolyline2D : public LandmarkBase
 		int getFirstId() const;
 		int getLastId() const;
 
-        const Eigen::VectorXs& getPointVector(int _i) const;
+        const Eigen::VectorXs getPointVector(int _i) const;
 
         StateBlockPtr getPointStateBlockPtr(int _i);