avformat/flac_picture: clear padding area

aviod use of uninitialized memory

Fixes: asan_heap-oob_1487fa4_4706_cov_364534849_cover_art.flac
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
