DO NOT MERGE Update libpng to 1.6.20

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

logerror(Image *image, const char *a1, const char *a2, const char *a3)
{
   fflush(stdout);
 if (image->image.warning_or_error)
      fprintf(stderr, "%s%s%s: %s\n", a1, a2, a3, image->image.message);

 else
      fprintf(stderr, "%s%s%s\n", a1, a2, a3);

 if (image->image.opaque != NULL)
 {
      fprintf(stderr, "%s: image opaque pointer non-NULL on error\n",
         image->file_name);
      png_image_free(&image->image);
 }

 return 0;
}

logerror(Image *image, const char *a1, const char *a2, const char *a3)
{
   fflush(stdout);
 if (image->image.warning_or_error)
      fprintf(stderr, "%s%s%s: %s\n", a1, a2, a3, image->image.message);

 else
      fprintf(stderr, "%s%s%s\n", a1, a2, a3);

 if (image->image.opaque != NULL)
 {
      fprintf(stderr, "%s: image opaque pointer non-NULL on error\n",
         image->file_name);
      png_image_free(&image->image);
 }

 return 0;
}
