avcodec/flacdec: Call ff_flacdsp_init() unconditionally

Fixes out of array access
Fixes: signal_sigsegv_324b135_3398_cov_246853371_short.flac
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
