scsi: libcxgbi: add a check for NULL pointer in cxgbi_check_route()

[ Upstream commit cc555759117e8349088e0c5d19f2f2a500bafdbd ]

ip_dev_find() can return NULL so add a check for NULL pointer.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>