webm_info,OutputCues: fix indexing of tracks

use GetTrackByIndex() rather than GetTrackByNumber(). the former is
0-based the latter is by track id, this loop iterates through all tracks
based on the track count.
fixes a segfault with tracks non-starting at 1 / increasing by 1.

BUG=webm:1415

Change-Id: I916328575d0871cf4090b1d123644d6f01e6ffb1
diff --git a/webm_info.cc b/webm_info.cc
index 2f9c00d..1e7dce3 100644
--- a/webm_info.cc
+++ b/webm_info.cc
@@ -1085,8 +1085,8 @@
   indent->Adjust(libwebm::kIncreaseIndent);
 
   do {
-    for (int track_num = 1; track_num <= num_tracks; ++track_num) {
-      const mkvparser::Track* const track = tracks.GetTrackByNumber(track_num);
+    for (int track_num = 0; track_num < num_tracks; ++track_num) {
+      const mkvparser::Track* const track = tracks.GetTrackByIndex(track_num);
       const mkvparser::CuePoint::TrackPosition* const track_pos =
           cue_point->Find(track);
 
@@ -1095,7 +1095,7 @@
             (track->GetType() == mkvparser::Track::kVideo) ? 'V' : 'A';
         fprintf(o, "%sCue Point:%d type:%c track:%d",
                 indent->indent_str().c_str(), cue_point_num, track_type,
-                track_num);
+                static_cast<int>(track->GetNumber()));
 
         if (options.output_seconds) {
           fprintf(o, " secs:%g",