ns can be NULL at this point, and my move of the check from
the original patch by Chaitanya broke this.

Fixes: 0ec84df4 ("nvme-core: check ctrl css before setting up zns")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>