Set a variable when you read the CRLF...

Code:
on *:SOCKREAD:membership: {
  if ($sockerr) halt
  else {
    while ($sock($sockname).rq) {
      sockread &binvar
      if (%now) bwrite membership.xml -1 -1 &binvar
      elseif ($bvar(&binvar,1-) == 13 10) set %now $true
    }
  }
}


You need to make sure you unset the %now variable later though... your best bet is probably to do it in the sockclose event, or whenever you close the socket.