Do a ScenePresence null check in HGMessageTransferModule.SendIMToScene() to stop a NullReferenceException being thrown if an HG IM is sent to a simulator running multiple regions

This is an attempt to address http://opensimulator.org/mantis/view.php?id=5791

OpenSim/Region/CoreModules/Avatar/InstantMessage/HGMessageTransferModule.cs

@@ -225,12 +225,13 @@ namespace OpenSim.Region.CoreModules.Avatar.InstantMessage
             foreach (Scene scene in m_Scenes)
             {
                 ScenePresence sp = scene.GetScenePresence(toAgentID);
-                if(!sp.IsChildAgent)
+                if(sp != null && !sp.IsChildAgent)
                 {
                     scene.EventManager.TriggerIncomingInstantMessage(gim);
                     successful = true;
                 }
             }
+
             if (!successful)
             {
                 // If the message can't be delivered to an agent, it

OpenSim/Region/Framework/Scenes/ScenePresence.cs

@@ -2854,6 +2854,8 @@ namespace OpenSim.Region.Framework.Scenes
                             Velocity = Vector3.Zero;
                             AbsolutePosition = pos;
 
+//                            m_log.DebugFormat("[SCENE PRESENCE]: Prevented flyoff for {0} at {1}", Name, AbsolutePosition);
+
                             AddToPhysicalScene(isFlying);
                         }
                     }