f (!(png_ptr->mode & PNG_HAVE_PLTE)) {
        /* Should be an error, but we can cope with it */
        png_warning(png_ptr, "Missing PLTE before tRNS");
    }
    else if (length > (png_uint_32)png_ptr->num_palette) {
        png_warning(png_ptr, "Incorrect tRNS chunk length");
        png_crc_finish(png_ptr, length);
        return;
    }
    ...
    png_crc_read(png_ptr, readbuf, (png_size_t)length);