Avoid NPE on shutdown if application doesn't use WebSocket

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1551481 13f79535-47bb-0310-9956-ffa450edef68
Avoid a warning that executor hasn't shutdown yet if the endpoint is not configured to wait for the executor to shutdown. (BIO is configured not to wait by default as threads in keep-alive can't be interrupted.)

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1551482 13f79535-47bb-0310-9956-ffa450edef68