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).
2 files changed