Replace strncpy call with memcpy when result is not NUL-terminated

Replace strncpy call with memcpy when result is not NUL-terminated

This fixes a new warning from GCC 8.1, -Wstringop-truncation:
https://gcc.gnu.org/gcc-8/changes.html
https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wstringop-truncation

Replacing with memcpy is what gcc suggests:
> As another example, the following call to strncpy results in copying
> to d just the characters preceding the terminating NUL, without
> appending the NUL to the end. Assuming the result of strncpy is
> necessarily a NUL-terminated string is a common mistake, and so the
> call is diagnosed. To avoid the warning when the result is not
> expected to be NUL-terminated, call memcpy instead.
>    void copy (char *d, const char *s)
>    {
>      strncpy (d, s, strlen (s));
>    }