Ensure Message.resultBox is nulled after successful async TryComplete

Since ResultBox.TryComplete will unwrap and recycle in the async use
case, we can no longer trust the resultBox reference and will want to
null it out.

StackExchange.Redis/StackExchange/Redis/Message.cs

@@ -501,6 +501,12 @@ public bool TryComplete(bool isAsync)
             if (resultBox != null)
             {
                 var ret = resultBox.TryComplete(isAsync);
+
+                if (ret && isAsync)
+                {
+                    resultBox = null; // in async mode TryComplete will have unwrapped and recycled resultBox; ensure we no longer reference it via this message
+                }
+
                 if (performance != null)
                 {
                     performance.SetCompleted();