alsdec: check block length

Fix writing over the end

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Addresses: CVE-2013-0845
(cherry picked from commit 2a0fb7286d67c47e44aa76c237ede117b22af616)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>