avformat/mov: clear padding area in mov_read_extradata()

Fixes: msan_uninit-mem_7f58816b71e3_7025_mov_svq3___svq3_weird_prediction2.mov
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>