fix(zset): correct the wrong calculation of range.maxex

[theyueli]

fixes #836

The example in #836 now produces the expected value:

127.0.0.1:6379> ZADD myzset 0 a 0 b 0 c 0 d 0 e 0 f 0 g
(integer) 7
127.0.0.1:6379> ZREVRANGEBYLEX myzset (c -
1) "b"
2) "a"