Prevent null pointer dereference.

Bug: 1072330
Change-Id: I0710128272b10adf7365235c865e47159950a458
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159472
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761071}

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

@@ -3412,6 +3412,8 @@ void AXNodeObject::ComputeAriaOwnsChildren(
 // last child: https://html.spec.whatwg.org/#the-figcaption-element
 static Element* GetChildFigcaption(const Node& node) {
   Element* element = ElementTraversal::FirstChild(node);
+  if (!element)
+    return nullptr;
   if (element->HasTagName(html_names::kFigcaptionTag))
     return element;