avformat/flac_picture: allocate buffer padding for picture

Fixes: heap array overread
Fixes: asan_heap-oob_14876d9_4706_cov_815472558_cover_art.flac
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
