use dictSlots for getting total slots number in dict (#7691)

src/dict.c

@@ -621,9 +621,7 @@ dictEntry *dictGetRandomKey(dict *d)
         do {
             /* We are sure there are no elements in indexes from 0
              * to rehashidx-1 */
-            h = d->rehashidx + (random() % (d->ht[0].size +
-                                            d->ht[1].size -
-                                            d->rehashidx));
+            h = d->rehashidx + (random() % (dictSlots(d) - d->rehashidx));
             he = (h >= d->ht[0].size) ? d->ht[1].table[h - d->ht[0].size] :
                                       d->ht[0].table[h];
         } while(he == NULL);