target/image: make i/j unsigned to avoid ubsan runtime error

target/image: make i/j unsigned to avoid ubsan runtime error

	src/target/image.c:1055:15: runtime error: left shift of 128 by 24 places cannot be represented in type 'int'

Change-Id: I322fd391cf3f242beffc8a274824763c8c5e69a4
Signed-off-by: Cody Schafer <openocd@codyps.com>
Reviewed-on: http://openocd.zylin.com/4584
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Christopher Head <chead@zaber.com>