fix(tabpage): correct check for failure to close window

Avoid closing window 999 times.

src/nvim/ex_docmd.c

@@ -6839,7 +6839,7 @@ void tabpage_close_other(tabpage_T *tp, int forceit)
 
     // Autocommands may delete the tab page under our fingers and we may
     // fail to close a window with a modified buffer.
-    if (!valid_tabpage(tp) || tp->tp_firstwin == wp) {
+    if (!valid_tabpage(tp) || tp->tp_lastwin == wp) {
       break;
     }
   }

test/functional/editor/tabpage_spec.lua

@@ -3,8 +3,10 @@ local helpers = require('test.functional.helpers')(after_each)
 local clear = helpers.clear
 local command = helpers.command
 local eq = helpers.eq
+local neq = helpers.neq
 local feed = helpers.feed
 local eval = helpers.eval
+local exec = helpers.exec
 
 describe('tabpage', function()
   before_each(clear)
@@ -34,5 +36,20 @@ describe('tabpage', function()
 
     eq(3, eval('tabpagenr()'))
   end)
+
+  it('does not crash or loop 999 times if BufWipeout autocommand switches window #17868', function()
+    exec([[
+      tabedit
+      let s:window_id = win_getid()
+      botright new
+      setlocal bufhidden=wipe
+      let g:win_closed = 0
+      autocmd WinClosed * let g:win_closed += 1
+      autocmd BufWipeout <buffer> call win_gotoid(s:window_id)
+      tabprevious
+      +tabclose
+    ]])
+    neq(999, eval('g:win_closed'))
+  end)
 end)