Convert strncpy to strlcpy
We try to avoid using strncpy() due to the ease of which it can
be misused. Convert this callsite to use strlcpy() instead to
match similar codepaths in this file.
Suggested-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/
2a796830-de2d-4030-b480-
d673f6cc5d94@eisentraut.org