jtag/tcl: Fix double free in jtag newtap argument parsing

jtag/tcl: Fix double free in jtag newtap argument parsing

Found when grepping for void* casts. I'm pretty sure there are about
*picking a number out of the blue* 0.31 bugs per pointer cast in OpenOCD.

Verified by valgrind, i.e.:
valgrind openocd -f interface/jtagkey.cfg -c "jtag newtap dummy cpu -irlen foo"

Change-Id: Idc12e8f18036cc908fdb572828fa46563be14242
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1665
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>