Fix a NULL-dereference Read Crash

Currently, trying to start selection autoscroll results in a crash if
layout object does not exist or is deleted.

This CL fixes this issue by adding a null check before trying to start
autoscroll for selection.

Bug: 1023249
Change-Id: I0bb81e865372077e2d34b95df4172c1283679201
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1912682Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Sarthak Shah <sarsha@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#714742}

third_party/blink/renderer/core/html/forms/html_select_element.cc

@@ -1581,12 +1581,14 @@ void HTMLSelectElement::ListBoxDefaultEventHandler(Event& event) {
         !mouse_event.ButtonDown())
       return;
 
-    if (LayoutObject* object = GetLayoutObject())
-      object->GetFrameView()->UpdateAllLifecyclePhasesExceptPaint();
+    LayoutObject* layout_object = GetLayoutObject();
+    if (layout_object) {
+      layout_object->GetFrameView()->UpdateAllLifecyclePhasesExceptPaint();
 
-    if (Page* page = GetDocument().GetPage()) {
-      page->GetAutoscrollController().StartAutoscrollForSelection(
-          GetLayoutObject());
+      if (Page* page = GetDocument().GetPage()) {
+        page->GetAutoscrollController().StartAutoscrollForSelection(
+            layout_object);
+      }
     }
     // Mousedown didn't happen in this element.
     if (last_on_change_selection_.IsEmpty())