avformat/mxfdec: check avio_read(UID) result

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fc9ba2fd98e_82_02785736.mxf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
