Fix memory leak where removing an NPC did not remove its circuits.

This was because we were removing by circuitcode where NPCs have no code.
Now removing by agent ID instead.
This commit also fixes the "show circuits" console command to work properly where the circuit has no associated IP address.

OpenSim/Region/Application/OpenSim.cs

@@ -1111,7 +1111,7 @@ namespace OpenSim
                             aCircuit.Name,
                             aCircuit.child ? "child" : "root",
                             aCircuit.circuitcode.ToString(),
-                            aCircuit.IPAddress.ToString(),
+                            aCircuit.IPAddress != null ? aCircuit.IPAddress.ToString() : "not set",
                             aCircuit.Viewer);
                 });