jtag/tcl: fix memory leak in error return

jtag/tcl: fix memory leak in error return

Clang static analyzer reported:
Warning:	line 196, column 3
Potential leak of memory pointed to by 'fields'

Free allocated memory pointed by 'fields' and fields[i].in_value

Change-Id: I0b3935d9a235544afc03e39a4648319047e65815
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5906
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>