Fix #244 access to freed memory in finalizer

I'm surprised this only happens on macOS.

I believe it is fixed as I tried >50 attempts
without reproducing the error again

source/fluid/future/stack.d

@@ -120,9 +120,17 @@ struct Stack(T) {
     out (; empty)
     do {
 
+        import core.memory : GC;
+
         // Already empty, nothing to do
         if (empty) return;
 
+        // If triggered from the GC, the objects cannot be trashed, as they may not be recyclable anymore
+        if (GC.inFinalizer) {
+            _top = null;
+            return;
+        }
+
         // Trash the bottom
         _bottom.next = _trash;
         _trash = _top;