Depending on the communication settings, mifare_desfire_read_data() may
write more than the provided "length" bytes to the "data" buffer,
possibly causing data corruption or crashes if no special care is taken.
Since the test suite is precisely a "no special care is taken" example,
assume only "length" bytes can be written to the "data" buffer and rely
on a temporary buffer for cryptographic operations.
Fixes issue 28.
