avformat/vqf: check avio_read()s return value more completely

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f180a523a71_5052_esvorbei_extd.vqf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
