ZOOKEEPER-4604: Creating a COMPLETION_STRING_STAT overrides acl_result completion

[adamyi]

c->c is a union so setting c.acl_result would override c.string_stat_result.

However, since they're all function pointers pointing to the same function, this won't cause any issue as c.string_stat_result is still correct. This might cause issue if we change the type of these completions in the future.

This also makes the code consistent.