Possible double-free

This commit:

https://github.com/YifangSun/ceph-rust/commit/d96bdd1b10dcffaa36a2637439cbf8492340bf23#diff-79f9ddb64f1be76f0a0837f3c47b12dddaf2b4aeed5e5e12cf93d42aabaf87abR473

Seems to be fixing double-free errors.
One should probably check whether they can actually happen, and if so, fix them in this main repo.
I'm not sure this is the appropriate way to fix them though. (The proposed "fix" seems to leave a potentially invalid object with a null ptr inside. Instead, functions that disconnect should probably take ownership, to avoid re-use of a disconnected client.)

@YifangSun