SCAN: when iterating ziplists or intsets always return cursor of 0.

The previous implementation assumed that the first call always happens
with cursor set to 0, this may not be the case, and we want to return 0
anyway otherwise the (broken) client code will loop forever.

src/db.c

@@ -462,6 +462,7 @@ void scanGenericCommand(redisClient *c, robj *o) {
 
         while(intsetGet(o->ptr,pos++,&ll))
             listAddNodeTail(keys,createStringObjectFromLongLong(ll));
+        cursor = 0;
     } else if (o->type == REDIS_HASH || o->type == REDIS_ZSET) {
         unsigned char *p = ziplistIndex(o->ptr,0);
         unsigned char *vstr;
@@ -475,6 +476,7 @@ void scanGenericCommand(redisClient *c, robj *o) {
                                  createStringObjectFromLongLong(vll));
             p = ziplistNext(o->ptr,p);
         }
+        cursor = 0;
     } else {
         redisPanic("Not handled encoding in SCAN.");
     }