[SCSI] libiscsi: check that command ptr is set before accessing it

If the scsi eh sends a TUR and the session is down we could
return SCSI_ML_HOST_BUSY. scsi eh will ignore this and send
ask us to abort the command and we blindly accesst the
command ptr.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>