patch 8.2.5079: DirChanged autocommand may use freed memory

Problem:    DirChanged autocommand may use freed memory. (Shane-XB Qian)
Solution:   Free the memory later. (closes #10555)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 24e2ed9..15aab48 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -2107,6 +2107,13 @@
   call assert_equal(expected, s:li)
   exe 'lcd ' .. fnameescape(s:dir_bar)
   call assert_equal(expected, s:li)
+
+  exe 'cd ' .. s:dir_foo
+  exe 'cd ' .. s:dir_bar
+  autocmd! test_dirchanged DirChanged global let g:result = expand("<afile>")
+  cd -
+  call assert_equal(s:dir_foo, g:result)
+
   call s:After_test_dirchanged()
 endfunc