avcodec/flashsv: check decode_hybrid() for overread

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f906558274e_268_14_244486_2009_04_28.flv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>