Core: Limit loop cycles while updating 'outlen'

There is an access violation risk by using 'while' loop to set
'outlen' in 4 'outvec' case. Use for loop to limit the cycles.

Change-Id: I983082c24b89b6ca21014b5b9e25007ba2cd1968
Signed-off-by: Ken Liu <ken.liu@arm.com>