libceph: check pointer before assigned to "c->rules[]"

It should be better to check pointer firstly, then assign it
to c->rules[]. Refine code a little bit.

Signed-off-by: Li Qiong <liqiong@nfschina.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>