patch 8.0.0486: crash and endless loop when closing windows in autocmd

Problem:    Crash and endless loop when closing windows in a SessionLoadPost
            autocommand.
Solution:   Check for valid tabpage.  (partly neovim #6308)
diff --git a/src/fileio.c b/src/fileio.c
index 32b2059..e743920 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -9033,6 +9033,11 @@
 	win_remove(curwin, NULL);
 	aucmd_win_used = FALSE;
 	last_status(FALSE);	    /* may need to remove last status line */
+
+	if (!valid_tabpage_win(curtab))
+	    /* no valid window in current tabpage */
+	    close_tabpage(curtab);
+
 	restore_snapshot(SNAP_AUCMD_IDX, FALSE);
 	(void)win_comp_pos();   /* recompute window positions */
 	unblock_autocmds();