examples: check the value returned by libusb_init()

Problem detected by the Coverity tool
CID 1042538 (#1 of 1): Unchecked return value (CHECKED_RETURN)4.
check_return: Calling function "libusb_init(libusb_context **)" without
checking return value (as is done elsewhere 11 out of 12 times).
examples: check the value returned by libusb_handle_events()

Problem detected by the Coverity tool
CID 1042539 (#1 of 1): Unchecked return value (CHECKED_RETURN)8.
check_return: Calling function "libusb_handle_events(libusb_context *)"
without checking return value (as is done elsewhere 6 out of 7 times).
examples: check value returned by libusb_bulk_transfer()

Problem detected by the Coverity tool
CID 1042540 (#1 of 1): Unchecked return value (CHECKED_RETURN)1.
check_return: Calling function "libusb_bulk_transfer(struct
libusb_device_handle *, unsigned char, unsigned char *, int, int *,
unsigned int)" without checking return value (as is done elsewhere 4 out
of 5 times).