[Mac] Fix a crash in handling keyboard shortcuts for empty bookmark folder menus.

chrome/browser/ui/cocoa/bookmarks/bookmark_bar_folder_controller.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "chrome/browser/ui/cocoa/bookmarks/bookmark_bar_folder_controller.h"
 
 #include <stddef.h>
 
 #include "base/mac/bundle_locations.h"
 #include "base/mac/sdk_forward_declarations.h"
@@ skipping 1617 matching lines @@
   if (index >= 0 && index < [self buttonCount])
     [[buttons_ objectAtIndex:index] highlight:state];
 }
 
 // Selects the required button and deselects the previously selected one.
 // An index of -1 means no selection.
 - (void)setSelectedButtonByIndex:(int)index {
   if (index == selectedIndex_)
     return;
 
+  if (index >= 0 && [[buttons_ objectAtIndex:index] isEmpty])
+    index = -1;
+
   [self setStateOfButtonByIndex:selectedIndex_ state:NO];
   [self setStateOfButtonByIndex:index state:YES];
   selectedIndex_ = index;
 
   [self showSelectedButton];
 }
 
 - (void)clearInputText {
   [typedPrefix_ release];
   typedPrefix_ = nil;
@@ skipping 451 matching lines @@
 
 - (void)setIgnoreAnimations:(BOOL)ignore {
   ignoreAnimations_ = ignore;
 }
 
 - (BookmarkButton*)buttonThatMouseIsIn {
   return buttonThatMouseIsIn_;
 }
 
 @end  // BookmarkBarFolderController