libctdb: check ctdb_request_free & ctdb_cancel used appropriately.

libctdb: check ctdb_request_free & ctdb_cancel used appropriately.

Since I made this mistake myself, we should check for it.

We could have one function that does both, but from a user's point of
view they are very different and it's quite possibly a bug if they
think the request is finished/unfinished when it's not.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>