Fix bug where a cloned object would report the wrong llGetNumberOfPrims() when avatars had been sitting on the original and a different avatar sat on the copy within the same opensim session.

This was because the sitting avatars list was being cloned rather than reset.
Addresses http://opensimulator.org/mantis/view.php?id=6649

OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs

@@ -1500,6 +1500,7 @@ namespace OpenSim.Region.Framework.Scenes
                 dupe.IsAttachment = true;
 
             dupe.AbsolutePosition = new Vector3(AbsolutePosition.X, AbsolutePosition.Y, AbsolutePosition.Z);
+            dupe.m_sittingAvatars = new List<UUID>();
 
             if (!userExposed)
             {