patch 9.0.0550: crash when closing a tabpage and buffer is NULL

Problem:    Crash when closing a tabpage and buffer is NULL.
Solution:   Adjust how autocommands are triggered when closing a window.
            (closes #11198, closes #11197)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 17bfd1b..3c95f87 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -515,6 +515,26 @@
   augroup! test-WinClosed
 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_switch_tab()
+  edit Xa
+  split Xb
+  split Xc
+  tab split
+  new
+  augroup test-WinClosed
+    autocmd WinClosed * tabprev | bwipe!
+  augroup END
+  close
+  " Check that the tabline has been fully removed
+  call assert_equal([1, 1], win_screenpos(0))
+
+  autocmd! test-WinClosed
+  augroup! test-WinClosed
+  %bwipe!
+endfunc
+
 func s:AddAnAutocmd()
   augroup vimBarTest
     au BufReadCmd * echo 'hello'