Cluster: be explicit about passing NULL as bind addr for connect.

The code was already correct but it was using that bindaddr[0] is set to
NULL as a side effect of current implementation if no bind address is
configured. This is not guarnteed to hold true in the future.

src/cluster.c

@@ -2421,7 +2421,8 @@ void clusterCron(void) {
             clusterLink *link;
 
             fd = anetTcpNonBlockBindConnect(server.neterr, node->ip,
-                node->port+REDIS_CLUSTER_PORT_INCR, server.bindaddr[0]);
+                node->port+REDIS_CLUSTER_PORT_INCR,
+                    server.bindaddr_count ? server.bindaddr[0] : NULL);
             if (fd == -1) {
                 redisLog(REDIS_DEBUG, "Unable to connect to "
                     "Cluster Node [%s]:%d -> %s", node->ip,