nvme_set_features() callers seem to expect that passing NULL as the
result pointer is acceptable.  Teach nvme_set_features() not to try to
write to the NULL address.

For symmetry, make the same change to nvme_get_features(), despite the
fact that all current callers pass a valid result pointer.

I assume that this bug hasn't been reported in practice because
the callers that pass NULL are all in the SCSI translation layer
and no one uses the relevant operations.

Cc: stable@vger.kernel.org
Signed-off-by: NAndy Lutomirski <luto@kernel.org>
Reviewed-by: NSagi Grimberg <sagi@grimberg.me>
Signed-off-by: NJens Axboe <axboe@fb.com>