Fix bug in SkipLast(0) for custom sequence (try to dequeue from empty queue)

Author: thibault-reigner

Ix.NET/Source/System.Linq.Async.Tests/System/Linq/Operators/SkipLast.cs

@@ -1,6 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT License.
-// See the LICENSE file in the project root for more information. 
+// See the LICENSE file in the project root for more information.
 
 using System;
 using System.Collections.Generic;
@@ -64,12 +64,13 @@ public async Task SkipLast_Zero()
         }
 
         [Fact]
-        public void SkipLast_Zero_NoAlias()
+        public async Task SkipLast_Zero_NoAlias()
         {
             var xs = Xs();
             var ys = xs.SkipLast(0);
 
             Assert.NotSame(xs, ys);
+            await NoNextAsync(ys.GetAsyncEnumerator());
         }
 
         private async IAsyncEnumerable<int> Xs()

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/SkipLast.cs

@@ -1,6 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT License.
-// See the LICENSE file in the project root for more information. 
+// See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -55,7 +55,8 @@ static async IAsyncEnumerable<TSource> Core(IAsyncEnumerable<TSource> source, in
                     {
                         do
                         {
-                            yield return queue.Dequeue();
+                            if (queue.Count > 0)
+                                yield return queue.Dequeue();
                             queue.Enqueue(e.Current);
                         }
                         while (await e.MoveNextAsync());