avcodec/flashsv: check avio_read() return in mov_read_udta_string()

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f3f90a8606a_3018_Sequence_1-Apple_ProRes_422_LT.mov
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>