Add(Audio|Video)Track: fix memory leak on failure

quiets a static analysis warning

Change-Id: Iaa6909e94deae81ace3e65f37ab746d72031620f
diff --git a/mkvmuxer/mkvmuxer.cc b/mkvmuxer/mkvmuxer.cc
index b7ab131..9bf1924 100644
--- a/mkvmuxer/mkvmuxer.cc
+++ b/mkvmuxer/mkvmuxer.cc
@@ -3361,7 +3361,10 @@
   track->set_width(width);
   track->set_height(height);
 
-  tracks_.AddTrack(track, number);
+  if (!tracks_.AddTrack(track, number)) {
+    delete track;
+    return 0;
+  }
   has_video_ = true;
 
   return track->number();
@@ -3401,7 +3404,10 @@
   track->set_sample_rate(sample_rate);
   track->set_channels(channels);
 
-  tracks_.AddTrack(track, number);
+  if (!tracks_.AddTrack(track, number)) {
+    delete track;
+    return 0;
+  }
 
   return track->number();
 }