avcodec/flacdec: check rice_order against blocksize

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f6e13c220d0_8489_short.flac
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>