OF-894: Do not attempt to close a session that is already closing.

src/java/org/jivesoftware/openfire/net/StalledSessionsFilter.java

@@ -47,6 +47,7 @@ public class StalledSessionsFilter extends IoFilterAdapter {
     @Override
 	public void filterWrite(NextFilter nextFilter, IoSession session, WriteRequest writeRequest)
             throws Exception {
+        if (!session.isClosing()) {
             // Get number of pending requests
             long pendingBytes = session.getScheduledWriteBytes();
             if (pendingBytes > bytesCap) {
@@ -59,6 +60,7 @@ public class StalledSessionsFilter extends IoFilterAdapter {
                 session.close(false);
                 throw new IOException("Closing session that seems to be stalled. Preventing OOM");
             }
+        }
         // Call next filter (everything is fine)
         super.filterWrite(nextFilter, session, writeRequest);
     }