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/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);
                         }
                     }