libceph: NULL deref on crush_decode() error path

If there is not enough space then ceph_decode_32_safe() does a goto bad.
We need to return an error code in that situation.  The current code
returns ERR_PTR(0) which is NULL.  The callers are not expecting that
and it results in a NULL dereference.

Fixes: f24e9980

 ("ceph: OSD client")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>