Prevent NPE if dispatch results in a nested call to process()

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1519501 13f79535-47bb-0310-9956-ffa450edef68
Ensure that when multiple dispatches are requested (e.g. one for write and one for read) if the first calls AsyncContext.complete() that the second dispatch is not processed.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1519508 13f79535-47bb-0310-9956-ffa450edef68