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)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index bcacaab..47e072e 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2085,6 +2085,7 @@
 		    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 @@
 			goto failed;
 		    }
 		    --ectx.ec_stack.ga_len;
-		    clear_tv(STACK_TV_BOT(-1));
-		    copy_tv(&li->li_tv, STACK_TV_BOT(-1));
+		    // Clear the list after getting the item, to avoid that it
+		    // make the item invalid.
+		    tv = STACK_TV_BOT(-1);
+		    temp_tv = *tv;
+		    copy_tv(&li->li_tv, tv);
+		    clear_tv(&temp_tv);
 		}
 		break;