TextTrackCue should check for non-NULL cue list pointer

Source/core/html/track/TextTrackCue.cpp

Index: Source/core/html/track/TextTrackCue.cpp
diff --git a/Source/core/html/track/TextTrackCue.cpp b/Source/core/html/track/TextTrackCue.cpp
index ddf2b9ac145fe97ff770dd01b3d12f6a3b09a7cc..b0c3d4ea26f0ee6795dff00031f5d0f4463e7c4d 100644
--- a/Source/core/html/track/TextTrackCue.cpp
+++ b/Source/core/html/track/TextTrackCue.cpp
@@ -479,8 +479,11 @@ void TextTrackCue::setText(const String& text)
 
 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;
 }