avformat/iff: check avio_read() return in get_metadata()

Fixes: msan_uninit-mem_7f9539ba8461_4760_dasboot_in_compressed
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>