Add IDENTIFY_SEND_CANCEL_RESULT state to send cancel command
after identify/verify completes, triggering firmware template update.
Implement cancel() callback for enroll and identify/verify cancellation.
Updated umockdev test data to cover the new flow.
When a USB transfer is cancelled, actual_length is set to -1. This
gets implicitly cast to gsize (unsigned) in log_transfer(), resulting
in a huge length passed to fp_dbg_hex_dump_data() and causing a
segfault.
Only dump data when the length is within valid bounds.
