Describe the bug
`
zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {

zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;

unsigned int rank[ZSKIPLIST_MAXLEVEL];
`
A short description of the bug.

To reproduce

Steps to reproduce the behavior and/or a minimal code sample.

Expected behavior
`
zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {

//---------------------------impovement-------------------------------------------

int level = zslRandomLevel(); // 1st obtains new level value

int max = max(level, zsl->level);

zskiplistNode *update[max]; // low probability to reach ZSKIPLIST_MAXLEVEL

//------------------------------------bug fix--------------------------------------------------------------
// from server.h
// unsigned long length; unsigned long span;
// #define ZSKIPLIST_MAXLEVEL 32 /* Should be enough for 2^64 elements /
// #define ZSKIPLIST_P 0.25 / Skiplist P = 1/4 */

// if x will be inserted at the end of list, and length has been greater than max of unsigned int, overflow can occur.

unsigned long rank[max];// long not int

`
A description of what you expected to happen.

Additional information

Any additional information that is relevant to the problem.