[debug] use istream::peek() != EOF to check eof of option-file

Because std::istream::operator bool() returns false AFTER FAILING get(),
the function `importOptions` read the final character TWICE.
Giving empty file for `--options=...` told me this error.

In this commit, `std::istream::peek()` is used instead. This member
function returns the next character without proceeding the file pointer,
which you can compare to EOF without any side-effects.