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