vim-patch:8.2.2474: using freed memory when window is closed by autocommand

Problem:    Using freed memory when window is closed by autocommand.
            (houyunsong)
Solution:   Check the window pointer is still valid.
2c7080bf1c

Add missing comment from Vim patch 8.0.1420.
Test fails.

src/nvim/quickfix.c

@@ -2313,7 +2313,10 @@ static bool qflist_valid(win_T *wp, unsigned int qf_id)
   qf_info_T *qi = &ql_info;
 
   if (wp) {
-    qi = GET_LOC_LIST(wp);
+    if (!win_valid(wp)) {
+      return false;
+    }
+    qi = GET_LOC_LIST(wp);  // Location list
     if (!qi) {
       return false;
     }

src/nvim/testdir/test_autocmd.vim

@@ -2741,6 +2741,19 @@ func Test_autocmd_closing_cmdwin()
   only
 endfunc
 
+func Test_autocmd_vimgrep()
+  augroup aucmd_vimgrep
+    au QuickfixCmdPre,BufNew,BufDelete,BufReadCmd * sb
+    au QuickfixCmdPre,BufNew,BufDelete,BufReadCmd * q9
+  augroup END
+  " TODO: if this is executed directly valgrind reports errors
+  call assert_fails('lv?a?', 'E926:')
+
+  augroup aucmd_vimgrep
+    au!
+  augroup END
+endfunc
+
 func Test_bufwipeout_changes_window()
   " This should not crash, but we don't have any expectations about what
   " happens, changing window in BufWipeout has unpredictable results.