null character outside of the byte region [sbuf, sbuf + len).
1) If the length of the buffer is 0, do not write a null character
at all. Previously, sockaddr_snprintf() may have been able to
overwrite sbuf[-1] if len was 0.
2) If the length of the buffer, len, is greater than 0, then write
the null at sbuf[len - 1]. Previously, sockaddr_snprintf()
wrote the null at buf[len - 1], where `buf' was a "cursor" that
did not necessarily equal `sbuf', the start of the buffer. Now,
sockaddr_snprintf() always writes the null at sbuf[len - 1].
