target: remove memory leaks

Found by clang.

Change-Id: Ifb25dca52f8d9e8e46a35f0947a7239f26eb3757
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2067


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Jörg Wunsch <openocd@uriah.heep.sax.de>