fix an endless loop (well, until you run out of storage from

tiny apr_pstrdup() calls and your machine crashes) when you
have a filter chain

  ap_getword() returns an empty string, not a NULL string,
  when there are no more words

fix a segfault when you don't have a filter chain

  ap_getword() does not check for a NULL string to search


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90810 13f79535-47bb-0310-9956-ffa450edef68