Followup to r1552080: avoid hard-coded download link

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1552257 13f79535-47bb-0310-9956-ffa450edef68
JspC passes configuration with ServletContext init params so a real implementation is required.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1552287 13f79535-47bb-0310-9956-ffa450edef68
Avoid possible NPE

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1552804 13f79535-47bb-0310-9956-ffa450edef68