avcodec/wmaprodec: Fix integer overflow in sfb_offsets initialization

Fixes out of array read
Fixes: asan_heap-oob_2aec5b0_1828_classical_22_16_2_16000_v3c_0_exclusive_0_29.wma
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>