Taking into account the code

fb(2, ar_coeff_lag);
num_pos_luma = 2 * current->ar_coeff_lag * (current->ar_coeff_lag + 1);
if (current->num_y_points)
    num_pos_chroma = num_pos_luma + 1;
else
    num_pos_chroma = num_pos_luma;

Max value for ar_coeff_lag is 3 (two bits), for num_pos_luma 24, and for
num_pos_chroma 25.

Both ar_coeffs_cb_plus_128 and ar_coeffs_cr_plus_128 may have up to
num_pos_chroma values.
Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>