[libpng16] Ensure png_ptr->trans_values is set in png_set_tRNS().

pngset.c

@@ -955,10 +955,11 @@ png_set_tRNS(png_structrp png_ptr, png_inforp info_ptr,
        if (num_trans > 0 && num_trans <= PNG_MAX_PALETTE_LENGTH)
        {
          /* Changed from num_trans to PNG_MAX_PALETTE_LENGTH in version 1.2.1 */
-          png_ptr->trans_alpha = info_ptr->trans_alpha = png_voidcast(png_bytep,
+          info_ptr->trans_alpha = png_voidcast(png_bytep,
              png_malloc(png_ptr, PNG_MAX_PALETTE_LENGTH));
           memcpy(info_ptr->trans_alpha, trans_alpha, (png_size_t)num_trans);
        }
+       png_ptr->trans_alpha = info_ptr->trans_alpha;
    }
 
    if (trans_color != NULL)