bug：exception handling

[xiongzhu666]

in boards_from_cornersr.cc if(!params.overlay) { // start from random index std::default_random_engine e; auto time = std::chrono::system_clock::now().time_since_epoch(); e.seed(static_cast(time.count())); if (corners.p.size() == 0) { return; } start = e() % corners.p.size(); }

when the corners.p.size() is zero， generate division by zero.....so need to add a judgment