(cp >= end) in channel_tls_process_netinfo_cell confuses analysis tools

David Fifield reports that:

  my_addr_len = (uint8_t) cell->payload[5];
  end = cell->payload + CELL_PAYLOAD_SIZE;
  cp = cell->payload + 6 + my_addr_len;
  if (cp >= end) {
    ...
    return;
  }

confuses the STACK analysis tool because the check could get optimized away. This check is indeed needless, since CELL_PAYLOAD_SIZE is 509, but UINT8_MAX + 6 is only 261, so that branch can never be taken. Shall we just remove it?