CrushWrapper:get_bucket: ret ENOENT for no bucket

CrushWrapper:get_bucket: ret ENOENT for no bucket

All the callers of CrushWrapper::get_bucket() check for error codes, but
not for NULL returns. So if there is no bucket (i.e., a NULL pointer) at
crush->bucket[i], just return the error code ENOENT. This is consistent
with how we handle other out-of-bounds requests.

Also, don't allow the caller to get us to try to access negative indices
in crush->bucket.

Signed-off-by: Colin McCabe <colinm@hq.newdream.net>