Put something in the ImprovedInstantMessage.BinaryBucket for llInstantMessage() to stop this crashing viewer 2.4.0 (1.23.5 was fine with this).

We're putting in a string of format "<region name><x pos floor int><y pos floor int><z pos floor int> which appears to be the expected value.
This resolves http://opensimulator.org/mantis/view.php?id=5356

OpenSim/Framework/Util.cs

@@ -1334,6 +1334,11 @@ namespace OpenSim.Framework
             return (ipaddr1 != null) ? "http://" + ipaddr1.ToString() + ":" + port1 : uri;
         }
 
+        public static byte[] StringToBytes256(string str, params object[] args)
+        {
+            return StringToBytes256(string.Format(str, args));
+        }
+        
         public static byte[] StringToBytes256(string str)
         {
             if (String.IsNullOrEmpty(str)) { return Utils.EmptyBytes; }
@@ -1352,6 +1357,11 @@ namespace OpenSim.Framework
             return data;
         }
 
+        public static byte[] StringToBytes1024(string str, params object[] args)
+        {
+            return StringToBytes1024(string.Format(str, args));
+        }
+        
         public static byte[] StringToBytes1024(string str)
         {
             if (String.IsNullOrEmpty(str)) { return Utils.EmptyBytes; }

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs

@@ -3063,12 +3063,19 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
             msg.ParentEstateID = 0; //ParentEstateID;
             msg.Position = Vector3.Zero;// new Vector3(m_host.AbsolutePosition);
             msg.RegionID = World.RegionInfo.RegionID.Guid;//RegionID.Guid;
-            msg.binaryBucket = new byte[0];// binaryBucket;
+            msg.binaryBucket 
+                = Util.StringToBytes256(
+                    "{0}/{1}/{2}/{3}", 
+                    World.RegionInfo.RegionName, 
+                    (int)Math.Floor(m_host.AbsolutePosition.X), 
+                    (int)Math.Floor(m_host.AbsolutePosition.Y), 
+                    (int)Math.Floor(m_host.AbsolutePosition.Z));
 
             if (m_TransferModule != null)
             {
                 m_TransferModule.SendInstantMessage(msg, delegate(bool success) {});
             }
+            
             ScriptSleep(2000);
       }