avcodec/takdec: always check bits_per_raw_sample

Fixes out of array access
Fixes: asan_heap-oob_19c7a94_6470_cov_1453611734_luckynight-partial.tak
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
