scsi: qla2xxx: Avoid that qla2x00_mem_free() crashes if called twice

Clear each pointer after having freed memory such that it becomes safe to
call qla2x00_mem_free() twice.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Cc: Giridhar Malavali <gmalavali@marvell.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>