Protect against possible NPE

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1539872 13f79535-47bb-0310-9956-ffa450edef68
Correctly implement delegation filtering for container packages.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1539873 13f79535-47bb-0310-9956-ffa450edef68
Correct r1539872 - thanks to Cyril Bonté

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1539881 13f79535-47bb-0310-9956-ffa450edef68
Make the magic constant a constant

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1539887 13f79535-47bb-0310-9956-ffa450edef68