Cluster: don't allow BY option of SORT as well.

There is the exception of a "constant" BY pattern that is used in order
to signal to don't sort at all. In this case no lookup is needed so it
is possible to support this case in Cluster mode.

src/sort.c

@@ -248,7 +248,17 @@ void sortCommand(redisClient *c) {
             sortby = c->argv[j+1];
             /* If the BY pattern does not contain '*', i.e. it is constant,
              * we don't need to sort nor to lookup the weight keys. */
-            if (strchr(c->argv[j+1]->ptr,'*') == NULL) dontsort = 1;
+            if (strchr(c->argv[j+1]->ptr,'*') == NULL) {
+                dontsort = 1;
+            } else {
+                /* If BY is specified with a real patter, we can't accept
+                 * it in cluster mode. */
+                if (server.cluster_enabled) {
+                    addReplyError(c,"BY option of SORT denied in Cluster mode.");
+                    syntax_error++;
+                    break;
+                }
+            }
             j++;
         } else if (!strcasecmp(c->argv[j]->ptr,"get") && leftargs >= 1) {
             if (server.cluster_enabled) {