patch 8.2.0937: asan failure in the flatten() test

Problem:    Asan failure in the flatten() test.
Solution:   Free the flattened list.
diff --git a/src/list.c b/src/list.c
index de802e6..675cf42 100644
--- a/src/list.c
+++ b/src/list.c
@@ -739,6 +739,7 @@
 list_flatten(list_T *list, long maxdepth)
 {
     listitem_T	*item;
+    listitem_T	*tofree;
     int		n;
 
     if (maxdepth == 0)
@@ -760,11 +761,14 @@
 	    vimlist_remove(list, item, item);
 	    if (list_extend(list, item->li_tv.vval.v_list, next) == FAIL)
 		return FAIL;
+	    clear_tv(&item->li_tv);
+	    tofree = item;
 
 	    if (item->li_prev == NULL)
 		item = list->lv_first;
 	    else
 		item = item->li_prev->li_next;
+	    list_free_item(list, tofree);
 
 	    if (++n >= maxdepth)
 	    {