Lock m_pendingObjects when calling GetPendingObjectsCount().

This is only called by a region console command.
We should also be locking m_partsUpdateQueue when dequeueing the next part, or locking m_pendingObjects in QueuePartForUpdate().
However, I won't do this now since I don't have time to analyze how this would affect liveness.

OpenSim/Region/Framework/Scenes/SceneViewer.cs

@@ -231,7 +231,8 @@ namespace OpenSim.Region.Framework.Scenes
         public int GetPendingObjectsCount()
         {
             if (m_pendingObjects != null)
-                return m_pendingObjects.Count;
+                lock (m_pendingObjects)
+                    return m_pendingObjects.Count;
 
             return 0;
         }