avformat/sierravmd: Check avio_read return value

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f82af392dae_1848_HR060606.VMD
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
