Change assertions to checks when parsing TrackPositions

Malformed files woudl cause the parser to assert.

Instead now it stops parsing and skips the segment.

Change-Id: I256b3b72e43f969693ae16547ca98da07d665e21

mkvparser.hpp

@@ -704,7 +704,7 @@ class CuePoint {
     // reference = clusters containing req'd referenced blocks
     //  reftime = timecode of the referenced block
 
-    void Parse(IMkvReader*, long long, long long);
+    bool Parse(IMkvReader*, long long, long long);
   };
 
   const TrackPosition* Find(const Track*) const;