Change assertions to checks when preloading Cues

If a malformed webm file contains invalid information on the Cues
the segment, the parser would assert.

Instead, now it ignores the Cues and skips the segment.

Change-Id: I9270f6a0935ce9e9a3765a5f324ae542c1ade1c7

mkvparser.hpp

@@ -758,7 +758,7 @@ class Cues {
   bool DoneParsing() const;
 
  private:
-  void Init() const;
+  bool Init() const;
   void PreloadCuePoint(long&, long long) const;
 
   mutable CuePoint** m_cue_points;