zdtm: don't write out of bounds

CID 174800 (#1 of 1): Out-of-bounds write (OVERRUN)
5. overrun-local: Overrunning array buf of 1024 bytes at byte offset 1024 using index n (which evaluates to 1024).
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/static/cgroup04.c

@@ -93,7 +93,7 @@ bool checkval(char *path, char *val)
 		return false;
 	}
 
-	n = read(fd, buf, sizeof(buf));
+	n = read(fd, buf, sizeof(buf) - 1);
 	close(fd);
 	if (n < 0) {
 		pr_perror("read");