Dead loop on aarch64 platform

[chestnutprog]

There are some tautological-constant-out-of-range-compare warnings on aarch64 platform, because char is an unsigned type on it. src/ext/cmd.cpp is fine because result is still correct in this situation, but src/ext/env.cpp doesn't work well.

external/boost/libs/process/src/ext/cmd.cpp:222:28: error: result of comparison of constant -1 with expression of type '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' (aka 'char') is always true [-Werror,-Wtautological-constant-out-of-range-compare]
    while (procargs.back() != EOF)
           ~~~~~~~~~~~~~~~ ^  ~~~
external/boost/libs/process/src/ext/cmd.cpp:240:25: error: result of comparison of constant -1 with expression of type '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' (aka 'char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
    if (procargs.back() == EOF)
        ~~~~~~~~~~~~~~~ ^  ~~~

external/boost/libs/process/src/ext/env.cpp:130:16: error: result of comparison of constant -1 with expression of type 'char' is always true [-Werror,-Wtautological-constant-out-of-range-compare]
    while (*nh != EOF)
           ~~~ ^  ~~~
external/boost/libs/process/src/ext/env.cpp:321:50: error: result of comparison of constant -1 with expression of type 'char' is always true [-Werror,-Wtautological-constant-out-of-range-compare]
    while (!procargs || procargs.get()[size - 1] != EOF)
                        ~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~

[klemens-morgenstern]

Is the fix to just cast EOF to char?

[chestnutprog]

Is the fix to just cast EOF to char?

I think it should be able to resolve the warning and dead loop. But I just noticed that Linux environment variables can actually include 0xff, so is it a more appropriate approach to record the actual length and use the length for judgment? I'm willing to submit a PR for this.