Fix lifetime-maintenance bug with std::string and c_str()

Fix lifetime-maintenance bug with std::string and c_str()

Here's more evidence that C++ is a nightmare to program---or that
I'm smart enough to realize that C++ is more clever than I will
ever be.

Most of my issues with C++ have to do with it hiding things from
me that I'd really like to and expect to be aware of as a C
programmer.

For example, the specific problem here is that there's a
short-lived std::string, from which I just want to copy
the C string. I try to do that on the next line, but before
I can, C++ has already called the destructor on the std::string.

Now, C++ isn't alone in doing garbage collecting like this.
But in a *real* garbage-collecting system, everything would
work that way. For example, here, I'm still holding a pointer
to the C string contents, so if the garbage collector were
aware of that reference, then it might clean up the std::string
container and leave the data I'm still using.

But that's not what we get with C++. Instead, some things are
reference counted and collected, (like the std::string), and
some things just aren't (like the C string it contains). The
end result is that it's very fragile. It forces me to be aware
of the timing of hidden functions. In a "real" system I wouldn't
have to be aware of that timing, and in C the function just
wouldn't be hidden.