Fix null race conditions in GameState.Close with local captures

ActiveChannel, ActiveClan, ActiveAccount, and GameAd could be set to
null by another thread between the null check and the method call.
Use local variable capture to prevent NullReferenceException.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wjlafrance

src/Atlasd/Battlenet/Protocols/Game/GameState.cs

@@ -19,7 +19,7 @@ public enum LogonTypes : UInt32
             NLS = 2,
         };
 
-        private bool IsDisposing = false;
+        private volatile bool IsDisposing = false;
 
         public ClientState Client { get; protected set; }
 
@@ -126,26 +126,29 @@ public static bool CanStatstringUpdate(Product.ProductCode code)
         public void Close()
         {
             // Remove this GameState from ActiveChannel
-            if (ActiveChannel != null)
+            var channel = ActiveChannel;
+            if (channel != null)
             {
-                ActiveChannel.RemoveUser(this); // will change this.ActiveChannel to null.
+                channel.RemoveUser(this); // will change this.ActiveChannel to null.
             }
 
             // Notify clan members
-            if (ActiveClan != null)
+            var clan = ActiveClan;
+            if (clan != null)
             {
-                ActiveClan.WriteStatusChange(this, false); // offline
+                clan.WriteStatusChange(this, false); // offline
             }
 
             // Update keys of ActiveAccount
-            if (ActiveAccount != null)
+            var account = ActiveAccount;
+            if (account != null)
             {
-                ActiveAccount.Set(Account.LastLogoffKey, DateTime.Now);
+                account.Set(Account.LastLogoffKey, DateTime.Now);
 
-                var timeLogged = (UInt32)ActiveAccount.Get(Account.TimeLoggedKey);
+                var timeLogged = (UInt32)account.Get(Account.TimeLoggedKey);
                 var diff = DateTime.Now - ConnectedTimestamp;
                 timeLogged += (UInt32)Math.Round(diff.TotalSeconds);
-                ActiveAccount.Set(Account.TimeLoggedKey, timeLogged);
+                account.Set(Account.TimeLoggedKey, timeLogged);
             }
 
             // Remove this OnlineName from ActiveAccounts and ActiveGameStates
@@ -156,7 +159,8 @@ public void Close()
             }
 
             // Remove this GameAd
-            if (GameAd != null && GameAd.RemoveClient(this)) GameAd = null;
+            var gameAd = GameAd;
+            if (gameAd != null && gameAd.RemoveClient(this)) GameAd = null;
         }
 
         public void Dispose() /* part of IDisposable */