Make sure shutdown_cont_event isn't holding any garbage references.

As it seems like there could be a race condition because the handling of
HTTP2_SESSION_EVENT_SHUTDOWN_INIT is going through the event loop again
and therefore may come in after the Session and its
associated contexts are already cleaned up thus causing an use-after-free possibility

(cherry picked from commit b0685db)

Author: sudheerv

proxy/http2/Http2ConnectionState.h

@@ -142,6 +142,7 @@ class Http2ConnectionState : public Continuation
   {
     if (shutdown_cont_event) {
       shutdown_cont_event->cancel();
+      shutdown_cont_event = nullptr;
     }
     cleanup_streams();