[libpng16] Fix the calculation of row_factor in png_check_chunk_length

(Bug report by Thuan Pham, SourceForge issue #278)
png_crc_read(png_structrp png_ptr, png_bytep buf, png_uint_32 length)
{
   if (png_ptr == NULL)
      return;

   png_read_data(png_ptr, buf, length);
   png_calculate_crc(png_ptr, buf, length);
}

png_crc_read(png_structrp png_ptr, png_bytep buf, png_uint_32 length)
{
   if (png_ptr == NULL)
      return;

   png_read_data(png_ptr, buf, length);
   png_calculate_crc(png_ptr, buf, length);
}
