This fixes a soundness bug in the recently added permutation argument:
To fingerprint a list $A = (a_1, ..., a_n)$ with selectors $S = (s_1,
..., s_n)$, we were computing:
$$
\prod_{i = 1}^n (X - s_i \cdot a_i)
$$
As a result, any element $a_i$ can be replaced with $0$, by setting the
selector to $0$!
Now, an element $i$ with $s_i = 0$ does not contribute to the product
anymore:
$$
\prod_{i = 1}^n (s_i \cdot (X - a_i - 1) + 1)
$$
