Prevent GCC warnings

[ikmsk10]

Some code of the fastcgipp has the following warnings:

../subprojects/fastcgi++/include/fastcgi++/protocol.hpp:366:29: warning: conversion to 'long unsigned int' from 'int' may change the sign of the result [-Wsign-conversion]
  366 |             uint8_t padding[paddingLength];
      |                             ^~~~~~~~~~~~~
In file included from ../src/core/application.hpp:7,
                 from ../src/core/route/handlers/graphql_handler.cpp:6:
../subprojects/fastcgi++/include/fastcgi++/manager.hpp:64:11: warning: 'class Fastcgipp::Manager_base' has virtual functions and accessible non-virtual destructor [-Wnon-virtual-dtor]
   64 |     class Manager_base

I'd prefer to remedy -Wnon-virtual-dtor, -Wsign-conversio, -Wunused-parameter corresponding code which given these warnings for possible using the library with the -pedantic and -Werror.