Added locking in AccessModule to prevent possible errors when shutting down a simulator that has >1 region

OpenSim/Region/CoreModules/World/Access/AccessModule.cs

@@ -90,13 +90,17 @@ namespace OpenSim.Region.CoreModules.World
         }
 
         public void AddRegion(Scene scene)
+        {
+            lock (m_SceneList)
             {
                 if (!m_SceneList.Contains(scene))
                     m_SceneList.Add(scene);
             }
+        }
 
         public void RemoveRegion(Scene scene)
         {
+            lock (m_SceneList)
                 m_SceneList.Remove(scene);
         }