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>
Merge remote-tracking branch 'qatar/release/9' into release/1.1

* qatar/release/9:
  shorten: Fix out-of-array read

Merged-by: Michael Niedermayer <michaelni@gmx.at>