avformat/isom: free extradata  on failure to read it

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f607d80f661_6965_mov00003.mqv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
