avcodec/pgssubdec: Use av_fast_padded_malloc() for rle buffer

Fixes: use of uninitialized memeory
Fixes: msan_uninit-mem_7fa421d0e222_1765_Girl_With_The_Dragon_Tattoo_2_23_56.mkv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
