TextTrackCue should check for non-NULL cue list pointer

Source/core/html/track/TextTrackCue.cpp

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  * Copyright (C) 2011, 2012, 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 461 matching lines @@
     cueWillChange();
     // Clear the document fragment but don't bother to create it again just yet as we can do that
     // when it is requested.
     m_webVTTNodeTree = 0;
     m_content = text;
     cueDidChange();
 }
 
 int TextTrackCue::cueIndex()
 {
-    if (m_cueIndex == invalidCueIndex)
-        m_cueIndex = track()->cues()->getCueIndex(this);
+    if (m_cueIndex == invalidCueIndex) {
+        TextTrackCueList* cues = track()->cues();
+        if (cues)
+            m_cueIndex = cues->getCueIndex(this);
+    }
 
     return m_cueIndex;
 }
 
 void TextTrackCue::invalidateCueIndex()
 {
     m_cueIndex = invalidCueIndex;
 }
 
 void TextTrackCue::createWebVTTNodeTree()
@@ skipping 688 matching lines @@
     if (m_cueSize != cue.size())
         return false;
     if (align() != cue.align())
         return false;
     
     return true;
 }
 
 } // namespace WebCore