Merge pull request #5 from HavretGC/failover_reconnect_issue

AMQNET-624: Fix failover issue when broker sends open frame and shortly after close frame
diff --git a/src/NMS.AMQP/Provider/Failover/FailoverProvider.cs b/src/NMS.AMQP/Provider/Failover/FailoverProvider.cs
index 18c1dee..12ab3e3 100644
--- a/src/NMS.AMQP/Provider/Failover/FailoverProvider.cs
+++ b/src/NMS.AMQP/Provider/Failover/FailoverProvider.cs
@@ -257,7 +257,7 @@
             {
                 try
                 {
-                    provider.Close();
+                    provider?.Close();
                 }
                 catch (Exception e)
                 {