Nitpicks to the previous commit.

Nitpicks to the previous commit.

We enforce by assertion that the target buffer size is nonzero before
subtracting 1 from it; the call to fatal() is replaced by another
assert so that it will give clearer diagnostic information if it
fails; the variable holding the return value of strlen should be
size_t and its declaration should be in a C90-compatible location.
Finally, the reason why the function needs to be exist is clarified.