vim-patch:8.0.0700: segfault with QuitPre autocommand closes the window

Problem:    Segfault with QuitPre autocommand closes the window. (Marek)
Solution:   Check that the window pointer is still valid. (Christian Brabandt,
            closes vim/vim#1817)

0ea5070d79

src/nvim/ex_docmd.c

@@ -5978,6 +5978,7 @@ static void ex_quit(exarg_T *eap)
   // Refuse to quit when locked or when the buffer in the last window is
   // being closed (can only happen in autocommands).
   if (curbuf_locked()
+      || !win_valid(wp)
       || (wp->w_buffer->b_nwindows == 1 && wp->w_buffer->b_locked > 0)) {
     return;
   }

src/nvim/testdir/test_tabpage.vim

@@ -473,5 +473,23 @@ func Test_tabnext_on_buf_unload2()
   endwhile
 endfunc
 
+func Test_close_on_quitpre()
+  " This once caused a crash
+  new
+  only
+  set bufhidden=delete
+  au QuitPre <buffer> close
+  tabnew tab1
+  tabnew tab2
+  1tabn
+  q!
+  call assert_equal(1, tabpagenr())
+  call assert_equal(2, tabpagenr('$'))
+  " clean up
+  while tabpagenr('$') > 1
+    bwipe!
+  endwhile
+  1b
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab