erp5_web: WebSite_login: make sure that there is a came_from
defined before using it as argument for isURLAncestorOf.
@vpelletier, this caused a bug, since empty came_from would crash isURLAncestorOf when urlparse is called for this url (https://lab.nexedi.com/nexedi/erp5/blob/master/product/ERP5Type/Core/Folder.py#L517). I think this is a way to solve, I do not know if isURLAncestorOf should be changed also to return something in such situation.