TCP: do not use an unknown status when the checksum is 0xffff
Otherwise it triggers an assert when adding the column as the field is defined as BASE_NONE and not BASE_DEC or BASE_HEX. Thus an unknown value (not in proto_checksum_vals[)array) cannot be represented. Mark the checksum as bad even if we process the packet. Closes #16816 Conflicts: epan/dissectors/packet-tcp.c