Use the messageLength field to indicate the message length.

Check it for sanity (and report errors), and use it to set the length of
the PTP packet tvbuff and the PTP top-level protocol item.

Bug: 10611
Change-Id: I9af6aad5f07d405ebb14885dc986a73e430d7bdb
Reviewed-on: https://code.wireshark.org/review/5967
Reviewed-by: Guy Harris <guy@alum.mit.edu>