This change makes it possible to build with newer versions of gcc
without warnings. There were two warnings issued:

* gcc 8 added
  [`-Wstringop-truncation`](https://developers.redhat.com/blog/2018/05/24/detecting-string-truncation-with-gcc-8/),
  which warns if a call to `strncpy(3)` is prone to accidentally
  truncating the destination string, since `strncpy(3)` does NOT add a
  terminating `NULL` if the destination buffer is not large enough to
  hold the input.

  This change uses the pattern suggested in
  https://us-cert.cisa.gov/bsi/articles/knowledge/coding-practices/strncpy-and-strncat
  to fix the locations flagged by gcc.
* There was a potentially uninitialized access of `dest` in `fs_copy`.