shorten: Fix out-of-array read

pred_order == FF_ARRAY_ELEMS(fixed_coeffs) is invalid too.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit 5f5ada3dbf97e306a74250ba8dcf8619ad59b020)
Signed-off-by: Tim Walker <tdskywalker@gmail.com>