Add test to verify that unused pointers are NULL.

The smartlist code takes great care to set all unused pointers inside
the smartlist memory to NULL. Check if this is also the case after
modifying the smartlist multiple times.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

src/test/test_containers.c

@@ -1006,6 +1006,7 @@ test_container_smartlist_remove(void *arg)
   tt_ptr_op(smartlist_get(sl, 1), OP_EQ, &array[2]);
   tt_ptr_op(smartlist_get(sl, 2), OP_EQ, &array[1]);
   tt_ptr_op(smartlist_get(sl, 3), OP_EQ, &array[2]);
+  tt_ptr_op(sl->list[4], OP_EQ, NULL);
 
  done:
   smartlist_free(sl);