avformat/mov: check avio_read() return in mov_read_dref()

Fixes: msan_uninit-mem_7f4960453a02_7264_mr_cork_jpeg.mov
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>