Replace "!" with debug assertion in "Permutations"

This adds to commit b31c7fd,
"Replace null-forgiving operator uses with debug assertions (morelinq#890)".

MoreLinq/Permutations.cs

@@ -103,7 +103,14 @@ public void Reset()
                 _hasMoreResults = true; // there's always at least one permutation: the original set itself
             }
 
-            public IList<T> Current => _current!;
+            public IList<T> Current
+            {
+                get
+                {
+                    Debug.Assert(_current is not null);
+                    return _current;
+                }
+            }
 
             object IEnumerator.Current => Current;