avformat/dxa: check avio_read() return

Fixes use of uninitialized memory
Fixes msan_uninit-mem_7fd4d4323ddd_2453_MUSOSP1.dxa
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
