The problem is that ecm_factor() might not find any factors, even if
there are some! So the line below is only probabilistic (=not good).
The solution is to first check if n_t is a prime (there are some
probabilistic but very good algorithms for that), and then if it is
not prime, keep calling this ecm_factor() until it gives a factor.
If it can't give it in let's say 10 iterations, we can raise an
exception.