fix zslRandomLevel - trivial but it has potential bug.

I think it is trivial and It might not be happened.

but it can be run with infinity loop when (random()&0xFFFF) < (ZSKIPLIST_P * 0xFFFF) is always True.

and it can be looped more than ZSKIPLIST_MAXLEVEL

```
while ((random()&0xFFFF) < (ZSKIPLIST_P * 0xFFFF)) 
```

so this patch will make it loop less or equals than ZSKIPLIST_MAXLEVEL