DO NOT MERGE Update libpng to 1.6.20

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

standard_text_validate(standard_display *dp, png_const_structp pp,
   png_infop pi, int check_end)
{
   png_textp tp = NULL;
   png_uint_32 num_text = png_get_text(pp, pi, &tp, NULL);

 if (num_text == 2 && tp != NULL)
 {
      standard_check_text(pp, tp, "image name", dp->ps->current->name);

 /* This exists because prior to 1.5.18 the progressive reader left the
       * png_struct z_stream unreset at the end of the image, so subsequent
       * attempts to use it simply returns Z_STREAM_END.
       */
 if (check_end)
         standard_check_text(pp, tp+1, "end marker", "end");
 }

 else
 {
 char msg[64];

      sprintf(msg, "expected two text items, got %lu",
 (unsigned long)num_text);
      png_error(pp, msg);
 }
}

standard_text_validate(standard_display *dp, png_const_structp pp,
   png_infop pi, int check_end)
{
   png_textp tp = NULL;
   png_uint_32 num_text = png_get_text(pp, pi, &tp, NULL);

 if (num_text == 2 && tp != NULL)
 {
      standard_check_text(pp, tp, "image name", dp->ps->current->name);

 /* This exists because prior to 1.5.18 the progressive reader left the
       * png_struct z_stream unreset at the end of the image, so subsequent
       * attempts to use it simply returns Z_STREAM_END.
       */
 if (check_end)
         standard_check_text(pp, tp+1, "end marker", "end");
 }

 else
 {
 char msg[64];

      sprintf(msg, "expected two text items, got %lu",
 (unsigned long)num_text);
      png_error(pp, msg);
 }
}
