Removed guard whether the channel is closed in checkIdleTimestamp().

This follows commit ff29a1cc51 for JETTY-1322.
The reason to remove the guard is that the channel may be closed multiple times, and if for any reason
a check to the idle timestamp triggers, then the idle callback is invoked, but it's not really idle: it is
already closed.
When used with SSL, this causes a truncation attack exception thrown by SSLEngine, because the idle
callback causes a shutdown of the SSLEngine without having received a SSL close alert.

jetty-io/src/main/java/org/eclipse/jetty/io/nio/SelectChannelEndPoint.java

@@ -280,7 +280,7 @@ public class SelectChannelEndPoint extends ChannelEndPoint implements AsyncEndPo
     public void checkIdleTimestamp(long now)
     {
         long idleTimestamp=_idleTimestamp;
-        if (!getChannel().isOpen() || idleTimestamp!=0 && _maxIdleTime>0 && now>(idleTimestamp+_maxIdleTime))
+        if (idleTimestamp!=0 && _maxIdleTime>0 && now>(idleTimestamp+_maxIdleTime))
         {
             onIdleExpired();
             _idleTimestamp=now;