Fix NPE in ClassByteClassLoader.addClass()

When org.jboss.remoting.marshal.MarshallerLoaderHandler gets a request for a class, it returns an instance of org.jboss.remoting.loading.ClassBytes. If MarshallerLoaderHandler isn't able to find the desired class, then the returned ClassBytes object has null value for the class byte array. However, org.jboss.remoting.loading.ClassByteClassLoader doesn't check for the possibility of a null byte array.