Fix race in ReconfigureAsync() snapshot creation

Though we lock on servers here, serverSnapshot is iterated in more cases, this reconfiguration iteration leaves null entries in the array causing downstream issues.

StackExchange.Redis/StackExchange/Redis/ConnectionMultiplexer.cs

@@ -1229,7 +1229,7 @@ internal async Task<bool> ReconfigureAsync(bool first, bool reconfigureAll, Text
                     int index = 0;
                     lock (servers)
                     {
-                        serverSnapshot = new ServerEndPoint[configuration.EndPoints.Count];
+                        var newSnapshot = new ServerEndPoint[configuration.EndPoints.Count];
                         foreach (var endpoint in configuration.EndPoints)
                         {
                             var server = (ServerEndPoint)servers[endpoint];
@@ -1242,8 +1242,9 @@ internal async Task<bool> ReconfigureAsync(bool first, bool reconfigureAll, Text
                                     servers.Add(endpoint, server);
                                 }
                             }
-                            serverSnapshot[index++] = server;
+                            newSnapshot[index++] = server;
                         }
+                        serverSnapshot = newSnapshot;
                     }
                     foreach (var server in serverSnapshot)
                     {