[Art-Net]
- The heuristic chek was missing "e" out of "Art-Net\0"
- tvb_length() - tvb_captured_length()

Change-Id: Iad9900f3a22e2fdbd0d6056efc1af90511cc458a
Reviewed-on: https://code.wireshark.org/review/5257
Reviewed-by: Anders Broman <a.broman58@gmail.com>