avcodec/pngdec: Fix paeth prediction with small images

Fixes out of array read
Fixes: asan_heap-oob_20b0a06_1962_cov_1907976991_delete_node_small.png
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
