Review of r1545580
Rephrase.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1545799 13f79535-47bb-0310-9956-ffa450edef68
Avoid NPEs if asyncContext is completed by listener

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1545801 13f79535-47bb-0310-9956-ffa450edef68