scsi: libcxgbi: fix NULL pointer dereference in cxgbi_device_destroy()

If cxgb4i_ddp_init() fails then cdev->cdev2ppm will be NULL, so add a check
for NULL pointer before dereferencing it.

Link: https://lore.kernel.org/r/1576676731-3068-1-git-send-email-varun@chelsio.com
Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>