[libpng16] Fix the calculation of row_factor in png_check_chunk_length

(Bug report by Thuan Pham, SourceForge issue #278)
png_crc_error(png_structrp png_ptr)
{
   png_byte crc_bytes[4];
   png_uint_32 crc;
   int need_crc = 1;

   if (PNG_CHUNK_ANCILLARY(png_ptr->chunk_name) != 0)
   {
      if ((png_ptr->flags & PNG_FLAG_CRC_ANCILLARY_MASK) ==
          (PNG_FLAG_CRC_ANCILLARY_USE | PNG_FLAG_CRC_ANCILLARY_NOWARN))
         need_crc = 0;
   }

   else /* critical */
   {
      if ((png_ptr->flags & PNG_FLAG_CRC_CRITICAL_IGNORE) != 0)
         need_crc = 0;
   }

#ifdef PNG_IO_STATE_SUPPORTED
   png_ptr->io_state = PNG_IO_READING | PNG_IO_CHUNK_CRC;
#endif

   /* The chunk CRC must be serialized in a single I/O call. */
   png_read_data(png_ptr, crc_bytes, 4);

   if (need_crc != 0)
   {
      crc = png_get_uint_32(crc_bytes);
      return ((int)(crc != png_ptr->crc));
   }

   else
      return (0);
}

png_crc_error(png_structrp png_ptr)
{
   png_byte crc_bytes[4];
   png_uint_32 crc;
   int need_crc = 1;

   if (PNG_CHUNK_ANCILLARY(png_ptr->chunk_name) != 0)
   {
      if ((png_ptr->flags & PNG_FLAG_CRC_ANCILLARY_MASK) ==
          (PNG_FLAG_CRC_ANCILLARY_USE | PNG_FLAG_CRC_ANCILLARY_NOWARN))
         need_crc = 0;
   }

   else /* critical */
   {
      if ((png_ptr->flags & PNG_FLAG_CRC_CRITICAL_IGNORE) != 0)
         need_crc = 0;
   }

#ifdef PNG_IO_STATE_SUPPORTED
   png_ptr->io_state = PNG_IO_READING | PNG_IO_CHUNK_CRC;
#endif

   /* The chunk CRC must be serialized in a single I/O call. */
   png_read_data(png_ptr, crc_bytes, 4);

   if (need_crc != 0)
   {
      crc = png_get_uint_32(crc_bytes);
      return ((int)(crc != png_ptr->crc));
   }

   else
      return (0);
}
