Fixes #4190 Jetty hangs after thread blocked in SharedBlockingCallback.block() called by HttpOutput.close.

Now releasing the buffer when in CLOSING state.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>

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

@@ -407,6 +407,7 @@ public class HttpOutput extends ServletOutputStream implements Runnable
                 {
                     if (!_state.compareAndSet(state, State.CLOSED))
                         break;
+                    releaseBuffer();
                     return;
                 }
                 case CLOSED: