[sass_values.cpp] Set the list length when making a new c_list.

This confused me and I don't know if it's intended behavior or not:

    union Sass_Value list = make_sass_list(my_length, sep);
    for (int i=0; i<my_length; i++) {
        ...
    }

That was populating the list but leaving the length not set inside the Sass_Value union. It seems that if we were passing in the length to make_sass_list() then that's probably the right place initialize the length in the union. It's probably unlikely that you'd want to pass in a length there but populate less than that (of course you're still free to do that).

-David