tasmlib_verifier_vm_proof_iter_dequeue_next_as_fripolynomial must reject polynomials with leading zero coefficients

[Sword-Smith]

The BFieldCodec implementation of Polynomial disallows trailing zeros in the coefficients list. A trailing zero in the coefficients list corresponds to a leading zero as in $p(x) = 0\cdot x^3 + x^2$. To make the host-machine implementation of the verifier consistent with the TASM version, we must reject these leading zeros.