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

commit cc555759 upstream.

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: Paul Gortmaker <paul.gortmaker@windriver.com>