Issue jetty#9476 Fixed null Callback usages in HttpConnection.SendCal…

…lback

1. Fixed the usages of `Callback` returned by `o.e.j.s.HttpConnection.SendCallback.release()` to check for `null` before invoking the `failedCallback(Callback, Throwable)` or `succeeded()` methods.

2. Also, added a try-finally block around the Callback invocations in order to ensure that shutdownOutput will get performed irrespective of whether they end up throwing any exception.

jetty-server/src/main/java/org/eclipse/jetty/server/HttpConnection.java

@@ -897,17 +897,27 @@ private void releaseChunk()
         @Override
         protected void onCompleteSuccess()
         {
-            release().succeeded();
-            if (_shutdownOut)
-                getEndPoint().shutdownOutput();
+            Callback callback = release();
+            try {
+                if (callback != null)
+                    callback.succeeded();
+            } finally {
+                if (_shutdownOut)
+                    getEndPoint().shutdownOutput();
+            }
         }
 
         @Override
         public void onCompleteFailure(final Throwable x)
         {
-            failedCallback(release(), x);
-            if (_shutdownOut)
-                getEndPoint().shutdownOutput();
+            Callback callback = release();
+            try {
+                if (callback != null)
+                    failedCallback(callback, x);
+            } finally {
+                if (_shutdownOut)
+                    getEndPoint().shutdownOutput();
+            }
         }
 
         @Override