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;
   }