Fix possible segfault on logicalunit update

When updating logicalunit with tgtdadm without any params
tgtd will segfault when calling strncmp on NULL.
The call to strsep later is ok and will return NULL.

To reproduce create a target with a lun and run:
tgtadm -m logicalunit -o update --tid 1 --lun 1

Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>