nvmet: auth: use NULL to clear a pointer in nvmet_auth_sq_free()

When compiling with C=1, the following sparse warning is generated:

auth.c:243:23: warning: Using plain integer as NULL pointer

Avoid this warning by using NULL to instead of 0 to set the sq tls_key
pointer.

Fixes: fa2e0f8b ("nvmet-tcp: support secure channel concatenation")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>