scsi: libfc: check fc_frame_payload_get() return value for null

We should not assume the payload of a PRLI or PLOGI respons is always
present.

Signed-off-by: Thomas Abraham <tabraham@suse.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Arun Easi <arun.easi@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>