Update torrentzip.go

As per specs http://www.pkware.com/documents/casestudies/APPNOTE.TXT section 4.5.3
This entry in the Local header MUST include BOTH original
      and compressed file size fields. If encrypting the 
      central directory and bit 13 of the general purpose bit
      flag is set indicating masking, the value stored in the
      Local Header for the original file size will be zero.