ziplist.c/pop: pop should return new allocated zl

Thanks to @mattsta 's [commit](https://github.com/mattsta/redis/commit/d4f1cf00fde28bd2dd3bca7d903d620bd40a0be7).
Find some little bugs in ziplist's test code(ziplist.c/pop).
pop() used ziplistDelete() which realloc zl.
So pop should return the new zl.

This P.R. will cause conflicts after mattsta's commit being merged.
I will fix the inconsistency if this little bug is worth being fixed. :)