Fix crash on GestureTap in Node::commonAncestor

Source/core/page/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 2247 matching lines @@
 
     // FIXME: Use a hit-test cache to avoid unnecessary hit tests. http://crbug.com/398920
     newHitTest = hitTestResultInFrame(m_frame, adjustedPoint, HitTestRequest::ReadOnly);
     PlatformMouseEvent fakeMouseUp(adjustedPoint, gestureEvent.globalPosition(),
         LeftButton, PlatformEvent::MouseReleased, gestureEvent.tapCount(),
         modifiers, PlatformMouseEvent::FromTouch,  gestureEvent.timestamp());
     bool swallowMouseUpEvent = !dispatchMouseEvent(EventTypeNames::mouseup, newHitTest.targetNode(), gestureEvent.tapCount(), fakeMouseUp, false);
 
     bool swallowClickEvent = false;
     if (m_clickNode) {
-        Node* clickTargetNode = newHitTest.targetNode()->commonAncestor(*m_clickNode, parentForClickEvent);
-        swallowClickEvent = !dispatchMouseEvent(EventTypeNames::click, clickTargetNode, gestureEvent.tapCount(), fakeMouseUp, true);
+        if (newHitTest.targetNode()) {
+            Node* clickTargetNode = newHitTest.targetNode()->commonAncestor(*m_clickNode, parentForClickEvent);
+            swallowClickEvent = !dispatchMouseEvent(EventTypeNames::click, clickTargetNode, gestureEvent.tapCount(), fakeMouseUp, true);
+        }
         m_clickNode = nullptr;
     }
 
     if (!swallowMouseUpEvent)
         swallowMouseUpEvent = handleMouseReleaseEvent(MouseEventWithHitTestResults(fakeMouseUp, newHitTest));
 
     return swallowMouseDownEvent | swallowMouseUpEvent | swallowClickEvent;
 }
 
 bool EventHandler::handleGestureLongPress(const GestureEventWithHitTestResults& targetedEvent)
@@ skipping 1572 matching lines @@
 unsigned EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return PlatformEvent::CtrlKey | PlatformEvent::AltKey;
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace blink