libusb: ignore string descs with index 0

When using the hid_get_{manufacturer,product,serial_number}_string functions, a device that has iManufacturer, iProduct or iSerialNumber equal to 0 would populate the buffer with the language ids in string descriptor 0. This patch makes the the functions return -1 instead. This seems to behave the same as the hidraw backend. String descriptor 0 can still be obtained via calling hid_get_indexed_string directly.