AX: Crash at WebCore::AccessibilityMenuList::addChildren()

AX: Crash at WebCore::AccessibilityMenuList::addChildren()
https://bugs.webkit.org/show_bug.cgi?id=144860

Reviewed by Mario Sanchez Prada.

Source/WebCore:

There were some unsafe pointer accesses in AccessibilityMenuList code that needed to be cleaned up.

Test: accessibility/menu-list-crash2.html

* accessibility/AccessibilityMenuList.cpp:
(WebCore::AccessibilityMenuList::addChildren):

LayoutTests:

* accessibility/menu-list-crash2-expected.txt: Added.
* accessibility/menu-list-crash2.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@184097 268f45cc-cd09-0410-ab3c-d52691b4dbfc