avformat/nutdec: check avio_read() return code

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f2785ab8669_6838_mewmew_vorbis_ssa.nut
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
