target: Fix memory leak

Change-Id: Ib23dfd653d8edacb890a46179e9d437c027d58e8
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/4048


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Chengyu Zheng <chengyu.zheng@polimi.it>
Reviewed-by: Andreas Färber <afaerber@suse.de>