libreboot-utils: simplify random tmpdir namegen

generalise it in rand.c because this logic will
be useful for other programs in the future.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/libreboot-utils/lottery.c

@@ -33,7 +33,7 @@ main(int argc, char **argv)
 
 	if (argc < 2) /* no spew */
 		spew_hex(buf, BUFSIZ);
-	free(buf);
+	free_and_set_null(&buf);
 
 	fprintf(stderr, "\n%s\n", same ? "You win!" : "You lose!");
 	return same ^ 1;