Check for an input event before showing a dialog box. (Bug#20813)

* lisp/subr.el (y-or-n-p):
* src/fns.c (Fyes_or_no_p): Check last-input-event as well
as last-nonmenu-event.

lisp/subr.el

@@ -2348,6 +2348,7 @@ is nil and `use-dialog-box' is non-nil."
 		  (t (setq temp-prompt (concat "Please answer y or n.  "
 					       prompt))))))))
      ((and (display-popup-menus-p)
+           last-input-event             ; not during startup
 	   (listp last-nonmenu-event)
 	   use-dialog-box)
       (setq prompt (funcall padded prompt t)

src/fns.c

@@ -2686,7 +2686,7 @@ if `last-nonmenu-event' is nil, and `use-dialog-box' is non-nil.  */)
   CHECK_STRING (prompt);
 
   if ((NILP (last_nonmenu_event) || CONSP (last_nonmenu_event))
-      && use_dialog_box)
+      && use_dialog_box && ! NILP (last_input_event))
     {
       Lisp_Object pane, menu, obj;
       redisplay_preserve_echo_area (4);