Make sure window is still valid in the middle of calling :lopen (#14240)

Make sure that oldwin is not invalid after splitting

Revisit this when porting vim patch v8.1.0892 and related quickfix patches.

src/nvim/quickfix.c

@@ -3617,6 +3617,15 @@ static int qf_open_new_cwindow(qf_info_T *qi, int height)
   if (win_split(height, flags) == FAIL) {
     return FAIL;  // not enough room for window
   }
+
+  // User autocommands may have invalidated the previous window after calling
+  // win_split, so add a check to ensure that the win is still here
+  if (IS_LL_STACK(qi) && !win_valid(win)) {
+    // close the window that was supposed to be for the loclist
+    win_close(curwin, false);
+    return FAIL;
+  }
+
   RESET_BINDING(curwin);
 
   if (IS_LL_STACK(qi)) {

src/nvim/window.c

@@ -1301,6 +1301,10 @@ int win_split_ins(int size, int flags, win_T *new_wp, int dir)
     p_wh = i;
   }
 
+  if (!win_valid(oldwin)) {
+    return FAIL;
+  }
+
   // Send the window positions to the UI
   oldwin->w_pos_changed = true;
 

test/functional/viml/errorlist_spec.lua

@@ -68,4 +68,17 @@ describe('setloclist()', function()
     command('lclose | wincmd w | lopen')
     eq('foo', get_cur_win_var('quickfix_title'))
   end)
+
+  it("doesn't crash when when window is closed in the middle #13721", function()
+    helpers.insert([[
+    hello world]])
+
+    command("vsplit")
+    command("autocmd WinLeave * :call nvim_win_close(0, v:true)")
+
+    command("call setloclist(0, [])")
+    command("lopen")
+
+    helpers.assert_alive()
+  end)
 end)