Fix possible null pointer in AXMenuListOption.

Bug: 908790
Change-Id: I7b8fe648f2190a83954d0eba08d636fa5055e6c6
Reviewed-on: https://chromium-review.googlesource.com/c/1352409
Commit-Queue: Katie Dektar <katie@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611411}

third_party/blink/renderer/modules/accessibility/ax_menu_list_option.cc

@@ -77,6 +77,8 @@ AXObject* AXMenuListOption::ComputeParent() const {
   if (!select)
     return nullptr;
   AXObject* select_ax_object = AXObjectCache().GetOrCreate(select);
+  if (!select_ax_object)
+    return nullptr;
 
   // This happens if the <select> is not rendered. Return it and move on.
   if (!select_ax_object->IsMenuList())