patch 8.2.2421: double free when using autocommand with "argdel"

Problem:    Double free when using autocommand with "argdel". (Houyunsong)
Solution:   Add the arglist_locked flag.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 9c996ec..dab6991 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -173,6 +173,12 @@
   quit
 endfunc
 
+func Test_argdelete_in_next()
+  au BufNew,BufEnter,BufLeave,BufWinEnter * argdel
+  call assert_fails('next a b', 'E1156:')
+  au! BufNew,BufEnter,BufLeave,BufWinEnter *
+endfunc
+
 func Test_autocmd_bufwinleave_with_tabfirst()
   tabedit
   augroup sample