avformat/mov: check avio_read return in mov_read_dvc1()

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f46d5199ee1_9456_vc1-wmapro.ism
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>