scsi: libfc: Check xid when looking up REC exchanges

We currently can only lookup the local xid, so we need
to reject REC with empty rxid.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>