Sentinel: reply -IDONTKNOW to get-master-addr-by-name on lack of info.

If we don't have any clue about a master since it never replied to INFO
so far, reply with an -IDONTKNOW error to SENTINEL
get-master-addr-by-name requests.

src/sentinel.c

@@ -2005,6 +2005,8 @@ void sentinelCommand(redisClient *c) {
         ri = sentinelGetMasterByName(c->argv[2]->ptr);
         if (ri == NULL) {
             addReply(c,shared.nullmultibulk);
+        } else if (ri->info_refresh == 0) {
+            addReplySds(c,sdsnew("-IDONTKNOW I have not enough information to reply. Please ask another Sentinel.\r\n"));
         } else {
             sentinelAddr *addr = ri->addr;