libusb: Fix null pointer dereference in hid_open()

Make sure hid_device_info.serial_number is not null before passing it to
wcscmp().

When libusb can't open a device for any reason (e.g. file system
permissions), hid_device_info.serial_number is NULL. Make sure
hid_device_info.serial_number is not null before passing it to wcscmp(3).

Signed-off-by: Michael Hanselmann <public@hansmi.ch>