posix/executor.hpp: write_error() narrowing conversion - Warning

Line 156 of boost_1_78_0/boost/process/detail/posix/executor.hpp was modified to use "auto" for the string length.
`const auto len = std::strlen(msg);`
The data[2] line however expects int.
```
int data[2] = {ec.value(), len + 1};
::write(_pipe_sink, &data[0], sizeof(int) * 2);

```
This leads to a narrowing warning like
`boost_1_78_0/boost/process/detail/posix/executor.hpp:156:40: error: narrowing conversion of '(((long unsigned int)len) + 1)' from 'long unsigned int' to 'int' inside { } [-Werror=narrowing]`