alsdec: fix number of decoded samples in first sub-block in BGMC mode.

Fixes CVE-2012-2790

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>
