This is a fairly subtle bug: We were implicitly creating a temporary std::string object and requesting c_str from this object.
This C string’s lifetime was that of the temporary std::string, and as we did a lot of std::string creation in the following line, with the new libc++, there was a good chance that the temporary string buffer would be re-used and so, our working directory would be set to a bogus value.
This closes issue #329.