libusb_helper: fix memory leak when no adapter is found

libusb_helper: fix memory leak when no adapter is found

When jtag_libusb_open() fails to find the adapter, it returns
error but left libusb initialized causing memory leak of libusb
internal data.
Issue found with valgrind when no adapter or board is connected to
the host, e.g.
	valgrind openocd -f board/st_nucleo_f4.cfg

Close the libusb operations if jtag_libusb_open() has to return
error.

Change-Id: Ieb2f110be15705dafe80c099e7d83c07056c2a41
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5701
Tested-by: jenkins