remove offset parameter from try_dissect_next_protocol()

Change-Id: I7452e6c5a9a24d44f83338aeb1dae389b87c8701
Reviewed-on: https://code.wireshark.org/review/3909
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>