fix: NetworkIdentity.SetClientOwner: overwriting the owner was still possible even though it shouldn't be. all caller functions double check and return early if it already has an owner, so we should do the same here.

Assets/Mirror/Runtime/NetworkIdentity.cs

@@ -206,10 +206,14 @@ internal set
         // used when adding players
         internal void SetClientOwner(NetworkConnection conn)
         {
+            // do nothing if it already has an owner
             if (connectionToClient != null && conn != connectionToClient)
             {
                 Debug.LogError($"Object {this} netId={netId} already has an owner. Use RemoveClientAuthority() first", this);
+                return;
             }
+
+            // otherwise set the owner connection
             connectionToClient = (NetworkConnectionToClient)conn;
         }