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>

lib/env.c

@@ -192,8 +192,7 @@ void set_env (int argc, char *const *argv)
 			}
 
 			if (NULL != *p) {
-				strncpy (variable, *argv, (size_t)(cp - *argv));
-				variable[cp - *argv] = '\0';
+				stpcpy(mempcpy(variable, *argv, (size_t)(cp - *argv)), "");
 				printf (_("You may not change $%s\n"),
 					variable);
 				continue;