patch 8.2.1134: Vim9: getting a list member may not work

Problem:    Vim9: getting a list member may not work.
Solution:   Clear the list only after copying the item. (closes #6393)

src/testdir/test_vim9_expr.vim

@@ -1141,6 +1141,11 @@ def Test_expr_member()
   call CheckDefExecFailure(["let d: dict<number>", "d = g:list_empty"], 'E1029: Expected dict but got list')
 enddef
 
+def Test_expr_index()
+  # getting the one member should clear the list only after getting the item
+  assert_equal('bbb', ['aaa', 'bbb', 'ccc'][1])
+enddef
+
 def Test_expr_member_vim9script()
   let lines =<< trim END
       vim9script

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1134,
 /**/
     1133,
 /**/

src/vim9execute.c

@@ -2085,6 +2085,7 @@ call_def_function(
 		    list_T	*list;
 		    varnumber_T	n;
 		    listitem_T	*li;
+		    typval_T	temp_tv;
 
 		    // list index: list is at stack-2, index at stack-1
 		    tv = STACK_TV_BOT(-2);
@@ -2109,8 +2110,12 @@ call_def_function(
 			goto failed;
 		    }
 		    --ectx.ec_stack.ga_len;
-		    clear_tv(STACK_TV_BOT(-1));
+		    // Clear the list after getting the item, to avoid that it
-		    copy_tv(&li->li_tv, STACK_TV_BOT(-1));
+		    // make the item invalid.
+		    tv = STACK_TV_BOT(-1);
+		    temp_tv = *tv;
+		    copy_tv(&li->li_tv, tv);
+		    clear_tv(&temp_tv);
 		}
 		break;