common: Overhaul the `sstrncpy` implementation.

common: Overhaul the `sstrncpy` implementation.

Properly check all arguments and behave in a sane manner, i.e. don't crash.

I went back and forth a few times on whether to return `NULL` or `dest` when `n == 0`.

*   On the one hand, `n == 0` is not really an error and a situation that could
    naturally occur, e.g. when you're implementing code that appends to the end
    of a string.
*   On the other hand, if we return `NULL` when `n` is zero we can guarantee
    that we will either return `NULL` or a null terminated string.

Ultimately I decided to go with the stronger guarantee, i.e.

```c
if (n == 0) {
  return NULL;
}
```