avcodec/pngenc: Replace memcpy by av_image_copy()

      Fixes out of array access
Fixes: 0cf176e6d3ab9fe924f39738e513f547/asan_generic_4a54aa_3431_aaa28be1cb32e307a9890cad06f84fba.avi

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>