avformat/isom: zero MPEG4AudioConfig to avoid leaving fields uninitialized

Fixes: use of uninitialized memory
Fixes: msan_uninit-mem_7f392c443e0d_1433_Crowd_Applause_01.caf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
