Updates after kkolinko review

      - Correct comment
- Use correct regular expression match (that makes regular expressions an even worse option)
- Improve (roughly x2) performance of invalid filename check

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc8.5.x/trunk@1809896 13f79535-47bb-0310-9956-ffa450edef68