DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)

static void initimage(Image *image, png_uint_32 opts, const char *file_name,
 int stride_extra)
{
   freeimage(image);
   memset(&image->image, 0, sizeof image->image);
   image->opts = opts;
   image->file_name = file_name;
   image->stride_extra = stride_extra;
}

static void initimage(Image *image, png_uint_32 opts, const char *file_name,
 int stride_extra)
{
   freeimage(image);
   memset(&image->image, 0, sizeof image->image);
   image->opts = opts;
   image->file_name = file_name;
   image->stride_extra = stride_extra;
}
