Fix limit undefined behavior crash in CLUSTER SLOT-STATS (#709)

We did not set a default value for limit, but it will be used
in addReplyOrderBy later, the undefined behavior may crash the
server since the value could be negative and crash will happen
in addReplyArrayLen.

An interesting reproducible example (limit reuses the value of -1):
```
> cluster slot-stats orderby key-count desc limit -1
(error) ERR Limit has to lie in between 1 and 16384 (maximum number of slots).
> cluster slot-stats orderby key-count desc
Error: Server closed the connection
```

Set the default value of limit to 16384.

---------

Signed-off-by: Binbin <[email protected]>

src/cluster_slot_stats.c

@@ -147,7 +147,7 @@ void clusterSlotStatsCommand(client *c) {
         }
         int i = 4; /* Next argument index, following ORDERBY */
         int limit_counter = 0, asc_desc_counter = 0;
-        long limit;
+        long limit = CLUSTER_SLOTS;
         while (i < c->argc) {
             int moreargs = c->argc > i + 1;
             if (!strcasecmp(c->argv[i]->ptr, "limit") && moreargs) {

tests/unit/cluster/slot-stats.tcl

@@ -199,7 +199,7 @@ start_cluster 1 0 {tags {external:skip cluster}} {
 # -----------------------------------------------------------------------------
 
 start_cluster 1 0 {tags {external:skip cluster}} {
-    
+
     # SET keys for target hashslots, to encourage ordering.
     set hash_tags [list 0 1 2 3 4]
     set num_keys 1
@@ -220,6 +220,7 @@ start_cluster 1 0 {tags {external:skip cluster}} {
 
     test "CLUSTER SLOT-STATS ORDERBY DESC correct ordering" {
         set orderby "key-count"
+        assert_error "ERR*" {R 0 CLUSTER SLOT-STATS ORDERBY $orderby DESC LIMIT -1}
         set slot_stats [R 0 CLUSTER SLOT-STATS ORDERBY $orderby DESC]
         assert_slot_stats_monotonic_descent $slot_stats $orderby
     }