scd: Handle a failure of libusb_init.

* scd/ccid-driver.c (ccid_get_reader_list, ccid_dev_scan): Handle
failure.

• Reported-by: Yuriy M. Kaminskiy <yumkam@gmail.com>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>