alsdec: check opt_order.

Fixes out of array write in quant_cof.
Also make sure no invalid opt_order stays in the context.

Fixes CVE-2012-2775

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>
