Folowup to r1430182: one more NPE to avoid when stopping a WebappLoader.
This line is absent in Tomcat 8.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1430536 13f79535-47bb-0310-9956-ffa450edef68
Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=54381
Add support for receiving pong messages.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1430538 13f79535-47bb-0310-9956-ffa450edef68