patch 9.0.0965: using one window for executing autocommands is insufficient

Problem:    Using one window for executing autocommands is insufficient.
Solution:   Use up to five windows for executing autocommands.

src/testdir/test_autocmd.vim

@@ -4068,4 +4068,27 @@ func Test_autocmd_split_dummy()
   call delete('Xerr')
 endfunc
 
+" This was crashing because there was only one window to execute autocommands
+" in.
+func Test_autocmd_nested_setbufvar()
+  CheckFeature python3
+
+  set hidden
+  edit Xaaa
+  edit Xbbb
+  call setline(1, 'bar')
+  enew
+  au BufWriteCmd Xbbb ++nested call setbufvar('Xaaa', '&ft', 'foo') | bw! Xaaa
+  au FileType foo call py3eval('vim.current.buffer.options["cindent"]')
+  wall
+
+  au! BufWriteCmd
+  au! FileType foo
+  set nohidden
+  call delete('Xaaa')
+  call delete('Xbbb')
+  %bwipe!
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab