+ Prevent NetState nullification recursion if the NetState is being disposed.

* This issue is responsible for causing client crashes when logging out, one of the signs is the sound of your backpack closing multiple times.
* Fix prevents multiple calls to subsystems such as the trade window, which had the potential to dupe items when returning them to the trade partner.
* Credit goes to UO:Revealed (www.uorevealed.com) for reporting the circumstances which lead to the fix, which seems to have been an overlooked issue for a very long time.

Server/Network/NetState.cs

@@ -1114,6 +1114,8 @@ namespace Server.Network {
 
 		private bool m_Disposing;
 
+		public bool IsDisposing { get { return m_Disposing; } }
+
 		public void Dispose() {
 			Dispose( true );
 		}