openSAFETY: Heuristic marks packet as truncated

 - There are cases, where the heuristic detection tries
   to access bytes, after a correct openSAFETY frame has
   been detected, leading to "Packet truncated" messages
   in the correct and complete detected packet

Change-Id: Ie389edf82144283ad2c15f0bf975066de01e3409
Reviewed-on: https://code.wireshark.org/review/4237
Reviewed-by: Roland Knall <rknall@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>