patch 9.1.0159: Crash in WinClosed after BufUnload closes other windows

Problem:  Crash in WinClosed after BufUnload closes other windows
Solution: Don't trigger WinClosed if the buffer is NULL (zeertzjq)

Now win_close_othertab() doesn't trigger any autocommands if the buffer
is NULL, so remove the autocmd blocking above (which was added not long
ago in patch v9.0.0550) for consistency.

Also remove an unreachable close_last_window_tabpage() above:
- It is only reached if only_one_window() returns TRUE and last_window()
  returns FALSE.
- If only_one_window() returns TRUE, there is only one tabpage.
- If there is only one tabpage and last_window() returns FALSE, the
  one_window() in last_window() must return FALSE, and the ONE_WINDOW
  in close_last_window_tabpage() must also be FALSE.
- So close_last_window_tabpage() doesn't do anything and returns FALSE.

Then the curtab != prev_curtab check also doesn't make much sense, and
the only_one_window() can be replaced with a check for popup and a call
to last_window() since this is a stricter check than only_one_window().

closes: #14166

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_autocmd.vim

@@ -819,6 +819,27 @@ func Test_WinClosed_switch_tab()
   %bwipe!
 endfunc
 
+" This used to trigger WinClosed twice for the same window, and the window's
+" buffer was NULL in the second autocommand.
+func Test_WinClosed_BufUnload_close_other()
+  tabnew
+  let g:tab = tabpagenr()
+  let g:buf = bufnr()
+  new
+  setlocal bufhidden=wipe
+  augroup test-WinClosed
+    autocmd BufUnload * ++once exe g:buf .. 'bwipe!'
+    autocmd WinClosed * call tabpagebuflist(g:tab)
+  augroup END
+  close
+
+  unlet g:tab
+  unlet g:buf
+  autocmd! test-WinClosed
+  augroup! test-WinClosed
+  %bwipe!
+endfunc
+
 func s:AddAnAutocmd()
   augroup vimBarTest
     au BufReadCmd * echo 'hello'