cleanup the add_cookie function a bit.

- the if(s) check is superfluid. s is guaranteed to be non-NULL
  (except for out of memory)
- strtok as late as possible to save some cycles.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100892 13f79535-47bb-0310-9956-ffa450edef68