lib/env.c: Replace strncpy(3) call by stpcpy(mempcpy(), "")

lib/env.c: Replace strncpy(3) call by stpcpy(mempcpy(), "")

We were using strncpy(3), which is designed to copy from a string into a
(null-padded) fixed-size character array.  However, we were doing the
opposite: copying from a known-size array (which was a prefix of a
string), into a string.  That's why we had to manually zero the buffer
afterwards.

Use instead mempcpy(3) to copy the non-null bytes, and then terminate
with a null byte with stpcpy(..., "").

Cc: "Serge E. Hallyn" <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>