minor: if a scene is already shutting down on Scene.Close(), warn and exit instead of attempting to run another shutdown concurrently.

Thanks to Oren Hurvitz for this change.

OpenSim/Region/Framework/Scenes/Scene.cs

@@ -1223,6 +1223,12 @@ namespace OpenSim.Region.Framework.Scenes
         // This is the method that shuts down the scene.
         public override void Close()
         {
+            if (m_shuttingDown)
+            {
+                m_log.WarnFormat("[SCENE]: Ignoring close request because already closing {0}", Name);
+                return;
+            }
+
             m_log.InfoFormat("[SCENE]: Closing down the single simulator: {0}", RegionInfo.RegionName);
 
             StatsReporter.Close();