Sentinel: fix address of master in Hello messages.

Once we switched configuration during a failover, we should advertise
the new address.

This was a serious race condition as the Sentinel performing the
failover for a moment advertised the old address with the new
configuration epoch: once trasmitted to the other Sentinels the broken
configuration would remain there forever, until the next failover
(because a greater configuration epoch is required to overwrite an older
one).

src/sentinel.c

@@ -1938,6 +1938,7 @@ void sentinelPingInstance(sentinelRedisInstance *ri) {
             char payload[REDIS_IP_STR_LEN+1024];
             sentinelRedisInstance *master = (ri->flags & SRI_MASTER) ?
                                              ri : ri->master;
+            sentinelAddr *master_addr = sentinelGetCurrentMasterAddress(master);
 
             snprintf(payload,sizeof(payload),
                 "%s,%d,%s,%d,%llu," /* Info about this sentinel. */
@@ -1946,7 +1947,7 @@ void sentinelPingInstance(sentinelRedisInstance *ri) {
                 (master->flags & SRI_CAN_FAILOVER) != 0,
                 (unsigned long long) sentinel.current_epoch,
                 /* --- */
-                master->name,master->addr->ip,master->addr->port,
+                master->name,master_addr->ip,master_addr->port,
                 master->config_epoch);
             retval = redisAsyncCommand(ri->cc,
                 sentinelPublishReplyCallback, NULL, "PUBLISH %s %s",