boost\process\pipe.hpp(282,21): warning C4244: 'argument': conversion from 'ptrdiff_t' to 'int', possible loss of data

I'm using [vcpkg](https://github.com/microsoft/vcpkg) and when I implement some code with `Boost.process`, I get the following compilation warning(s):
```
1>C:\vcpkg\installed\x64-windows\include\boost\process\pipe.hpp(282,21): warning C4244: 'argument': conversion from 'ptrdiff_t' to 'int', possible loss of data
1>C:\vcpkg\installed\x64-windows\include\boost\process\pipe.hpp(263): message : while compiling class template member function 'bool boost::process::basic_pipebuf>::_write_impl(void)'
1>C:\vcpkg\installed\x64-windows\include\boost\process\pipe.hpp(191): message : see reference to function template instantiation 'bool boost::process::basic_pipebuf>::_write_impl(void)' being compiled
1>C:\vcpkg\installed\x64-windows\include\boost\process\pipe.hpp(300): message : see reference to class template instantiation 'boost::process::basic_pipebuf>' being compiled
1>D:\Cpp\MyApplication\Main.cpp(37): message : see reference to class template instantiation 'boost::process::basic_ipstream>' being compiled
```

The code in question can e.g. easily be fixed by adding an additional explicit `int` cast:

```
bool _write_impl()
{
if (!_pipe.is_open())
return false;

auto base = this->pbase();

if (base == this->pptr())
return true;

std::ptrdiff_t wrt = _pipe.write(base,
static_cast(this->pptr() - base));

std::ptrdiff_t diff = this->pptr() - base;

if (wrt < diff)
std::move(base + wrt, base + diff, base);
else if (wrt == 0) //broken pipe
return false;

this->pbump((int) - wrt); // <---

return true;
}
```