The .mkv parser lib leaves some tracks NULL if it doesn't support them.

But it still counts them as valid tracks...

Change-Id: I77e8fd24cce91a586e8e418759db55b6eba449c0
related-to-bug: 3377186

media/libstagefright/matroska/MatroskaExtractor.cpp

@@ -707,6 +707,12 @@ void MatroskaExtractor::addTracks() {
     for (size_t index = 0; index < tracks->GetTracksCount(); ++index) {
         const mkvparser::Track *track = tracks->GetTrackByIndex(index);
 
+        if (track == NULL) {
+            // Apparently this is currently valid (if unexpected) behaviour
+            // of the mkv parser lib.
+            continue;
+        }
+
         const char *const codecID = track->GetCodecId();
         LOGV("codec id = %s", codecID);
         LOGV("codec name = %s", track->GetCodecNameAsUTF8());