Cluster: properly send ping to nodes not pinged foro too much time.

In commit d728ec6d it was introduced the concept of sending a ping to
every node not receiving a ping since node_timeout/2 seconds.
However the code was located in a place that was not executed because of
a previous conditional causing the loop to re-iterate.

This caused false positives in nodes availability detection.

The current code is still not perfect as a node may be detected to be in
PFAIL state even if it does not reply for just node_timeout/2 seconds
that is not correct. There is a plan to improve this code ASAP.

src/cluster.c

@@ -1555,11 +1555,6 @@ void clusterCron(void) {
             (REDIS_NODE_MYSELF|REDIS_NODE_NOADDR|REDIS_NODE_HANDSHAKE))
                 continue;
 
-        /* Check only if we already sent a ping and did not received
-         * a reply yet. */
-        if (node->ping_sent == 0 ||
-            node->ping_sent <= node->pong_received) continue;
-
         /* If our ping is older than half the cluster timeout (may happen
          * in a cluster with many nodes), send a new ping. */
         if (node->link &&
@@ -1569,6 +1564,11 @@ void clusterCron(void) {
             continue;
         }
 
+        /* Check only if we already sent a ping and did not received
+         * a reply yet. */
+        if (node->ping_sent == 0 ||
+            node->ping_sent <= node->pong_received) continue;
+
         /* If we never received a pong, use the ping time to compute
          * the delay. */
         if (node->pong_received) {