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/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 3bf5780..aef1815 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1141,6 +1141,11 @@
   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