prores: Error out only on surely incomplete ac_coeffs

(cherry picked from commit 2df7f7714a12a59d31058aba15fb1e348e36b0ab)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Merge remote-tracking branch 'qatar/release/9' into release/1.1

* qatar/release/9:
  prores: Error out only on surely incomplete ac_coeffs

Merged-by: Michael Niedermayer <michaelni@gmx.at>
update for 1.1.8

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