fix(buffer): disable buffer-updates before removing buffer from window

There can be other places that access window buffer info (e.g.
`tabpagebuflist()`), so checking `w_closing` in `win_findbuf()` doesn't
solve the crash in all cases, and may also cause Nvim's behavior to
diverge from Vim.

(cherry picked from commit fa30ef1c31)

src/nvim/buffer.c

@@ -578,6 +578,10 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
     return false;
   }
 
+  // Disable buffer-updates for the current buffer.
+  // No need to check `unload_buf`: in that case the function returned above.
+  buf_updates_unload(buf, false);
+
   if (win != NULL  // Avoid bogus clang warning.
       && win_valid_any_tab(win)
       && win->w_buffer == buf) {
@@ -590,10 +594,6 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
     buf->b_nwindows--;
   }
 
-  // Disable buffer-updates for the current buffer.
-  // No need to check `unload_buf`: in that case the function returned above.
-  buf_updates_unload(buf, false);
-
   // Remove the buffer from the list.
   if (wipe_buf) {
     // Do not wipe out the buffer if it is used in a window.

src/nvim/window.c

@@ -7207,7 +7207,7 @@ void win_findbuf(typval_T *argvars, list_T *list)
   int bufnr = tv_get_number(&argvars[0]);
 
   FOR_ALL_TAB_WINDOWS(tp, wp) {
-    if (!wp->w_closing && wp->w_buffer->b_fnum == bufnr) {
+    if (wp->w_buffer->b_fnum == bufnr) {
       tv_list_append_number(list, wp->handle);
     }
   }

test/functional/lua/buffer_updates_spec.lua

@@ -252,9 +252,8 @@ describe('lua buffer event callbacks: on_lines', function()
     eq(2, meths.win_get_cursor(0)[1])
   end)
 
-  it('does not SEGFAULT when calling win_findbuf in on_detach', function()
+  it('does not SEGFAULT when accessing window buffer info in on_detach #14998', function()
-
+    local code = [[
-    exec_lua[[
       local buf = vim.api.nvim_create_buf(false, false)
 
       vim.cmd"split"
@@ -262,13 +261,19 @@ describe('lua buffer event callbacks: on_lines', function()
 
       vim.api.nvim_buf_attach(buf, false, {
         on_detach = function(_, buf)
+          vim.fn.tabpagebuflist()
           vim.fn.win_findbuf(buf)
         end
       })
     ]]
 
+    exec_lua(code)
     command("q!")
     helpers.assert_alive()
+
+    exec_lua(code)
+    command("bd!")
+    helpers.assert_alive()
   end)
 
   it('#12718 lnume', function()