Sentinel: check for disconnected links in sentinelSendHello().

Does not fix any bug as the test is performed by the caller, but better
to have the check.

src/sentinel.c

@@ -2069,6 +2069,7 @@ int sentinelSendHello(sentinelRedisInstance *ri) {
 
     /* Try to obtain our own IP address. */
     if (anetSockName(ri->cc->c.fd,ip,sizeof(ip),NULL) == -1) return REDIS_ERR;
+    if (ri->flags & SRI_DISCONNECTED) return;
 
     /* Format and send the Hello message. */
     snprintf(payload,sizeof(payload),