correct a null pointer exception check before it gets caught and logged as an error

apps/streaming/java/src/net/i2p/client/streaming/MessageHandler.java

@@ -73,8 +73,8 @@ public class MessageHandler implements I2PSessionListener {
      * @param session that has been terminated
      */
     public void disconnected(I2PSession session) {
-        if (_log.shouldLog(Log.ERROR))
-            _log.error("I2PSession disconnected");
+        if (_log.shouldLog(Log.WARN))
+            _log.warn("I2PSession disconnected");
         _manager.disconnectAllHard();
         
         List listeners = null;