Gracefully handle track-less elements on updating track selection.

Source/core/html/HTMLMediaElement.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 3269 matching lines @@
 void HTMLMediaElement::automaticTrackSelectionForUpdatedUserPreference()
 {
     markCaptionAndSubtitleTracksAsUnconfigured();
     m_processingPreferenceChange = true;
     m_closedCaptionsVisible = false;
     honorUserPreferencesForAutomaticTextTrackSelection();
     m_processingPreferenceChange = false;
 
     // If a track is set to 'showing' post performing automatic track selection,
     // set closed captions state to visible to update the CC button and display the track.
-    m_closedCaptionsVisible = m_textTracks->hasShowingTracks();
+    if (m_textTracks)
+        m_closedCaptionsVisible = m_textTracks->hasShowingTracks();
     updateTextTrackDisplay();
 }
 
 void HTMLMediaElement::markCaptionAndSubtitleTracksAsUnconfigured()
 {
     if (!m_textTracks)
         return;
 
     // Mark all tracks as not "configured" so that
     // honorUserPreferencesForAutomaticTextTrackSelection() will reconsider
@@ skipping 386 matching lines @@
 
 #if ENABLE(WEB_AUDIO)
 void HTMLMediaElement::clearWeakMembers(Visitor* visitor)
 {
     if (!Heap::isHeapObjectAlive(m_audioSourceNode) && audioSourceProvider())
         audioSourceProvider()->setClient(nullptr);
 }
 #endif
 
 }