avformat/siff: check avio_read() return value before returning packet

Fixes: /ld/michael/ffmpeg_uninit/ffmpeg_uninit/done/msan_uninit-mem_7fb3e0fa86e9_1980_INTRO_B.VB
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
