Cluster: use CountKeysInSlot() when we just need the count.

src/cluster.c

@@ -1534,13 +1534,7 @@ void clusterCommand(redisClient *c) {
             if (server.cluster->slots[slot] == server.cluster->myself &&
                 n != server.cluster->myself)
             {
-                int numkeys;
-                robj **keys;
-
-                keys = zmalloc(sizeof(robj*)*1);
-                numkeys = GetKeysInSlot(slot, keys, 1);
-                zfree(keys);
-                if (numkeys != 0) {
+                if (CountKeysInSlot(slot) != 0) {
                     addReplyErrorFormat(c, "Can't assign hashslot %d to a different node while I still hold keys for this hash slot.", slot);
                     return;
                 }

src/redis.h

@@ -1227,6 +1227,7 @@ int selectDb(redisClient *c, int id);
 void signalModifiedKey(redisDb *db, robj *key);
 void signalFlushedDb(int dbid);
 unsigned int GetKeysInSlot(unsigned int hashslot, robj **keys, unsigned int count);
+unsigned int CountKeysInSlot(unsigned int hashslot);
 int verifyClusterConfigWithData(void);
 
 /* API to get key arguments from commands */