tests: avoid use of freed pointer

[spotted by coverity]

This is only in tests/, but easy to fix, so...
I've included extra context so you can see how var->value would
be used in the following atoi call.

From cf9ae83fd2da8d7a289b048ef0feed4096f6d263 Mon Sep 17 00:00:00 2001

From: Jim Meyering <meyering@redhat.com>
Date: Tue, 20 Sep 2011 16:32:59 +0200
Subject: [PATCH] avoid use of free'd pointer

• asschk.c (set_type_var): Set var->value to NULL after freeing it,

to avoid subsequent use of freed pointer.