Fix CancellationTokenRegistration.Unregister race condition (#27949)

Author: stephentoub

src/System.Private.CoreLib/shared/System/Threading/CancellationTokenSource.cs

@@ -524,6 +524,7 @@ internal CancellationTokenRegistration InternalRegister(
                 {
                     // Assign the next available unique ID.
                     id = partition.NextAvailableId++;
+                    Debug.Assert(id != 0, "IDs should never be the reserved value 0.");
 
                     // Get a node, from the free list if possible or else a new one.
                     node = partition.FreeNodeList;
@@ -942,9 +943,16 @@ public CallbackPartition(CancellationTokenSource source)
 
             internal bool Unregister(long id, CallbackNode node)
             {
-                Debug.Assert(id != 0, "Expected non-zero id");
                 Debug.Assert(node != null, "Expected non-null node");
 
+                if (id == 0)
+                {
+                    // In general, we won't get 0 passed in here.  However, race conditions between threads
+                    // Unregistering and also zero'ing out the CancellationTokenRegistration could cause 0
+                    // to be passed in here, in which case there's nothing to do. 0 is never a valid id.
+                    return false;
+                }
+
                 bool lockTaken = false;
                 Lock.Enter(ref lockTaken);
                 try