Fix error when path is extended

After inserting nodes to a path with lpe effect, method doBeforeEffect is called a couple of times. On the first time there is an inconsistency between the number of elements in nodesatellites[i] vector and the value returned by the function count_path_nodes(), so it cannot be used as an index to access the vector. I changed it to use .back() to access the last item, and just to keep code consistent I also used .front() to access the first item.

fix: #4719 (closed)